I would argue the superior Cast Speed will always win. The main reason is that most bosses force you to move regularly, hence you get limited time to stand there and just cast away as you did in your tests. So for the limited time you get to stand and cast, the more Fireballs (or whatever) you can squeeze out the more Ignite.
Sheesh. Neither did I, 
Just to pick up on this… the use of Elemental Nova is only recommended if you are proccing it automatically with Frost Claw. You don’t cast it directly.
That said, I think you should definitely try swapping to Frost Claw instead of FB and proccing your Nova that way, it is absolutely excellent at stacking Ignite fast. You will have Frost Claw casting super fast, hitting many more targets than FB will, plus automatic Novas going off all over the place too. This is almost like lots of extra free Ignite stacks just for casting a single spell. You will have Mana issues initially I suspect but they are easily fixed.
This is the one I’m using. You’re most likely correct I just went with the idea that as I add stuff in other places the Cast Speed will prove to have some sort of advantage outside of just the numbers themselves.
Do you spec into both Frost Claw and EN or do you just use FC to proc your basic EN for the burst spread effect. Not sure if I can replace two skills (i.e. Fireball and ?) to use both FC and EN.
I did try FC, it just had way too much mana drain given my auto procs in Flame Rush. And I wasn’t keen on the feel of the spell. How did you spec FC? I went with mostly stuff I think on the right and lower right of the tree with a dip into the upper right quadrant.
for FC, you basically always have to get all the -mana cost available to make it usable. There is a -3 mana cost on the left, did you try that?
On speccing, try things out! I have the feeling that Runic invocation does not contribute much to your damage anyway so you could try to use an unspecced version, if I recall correctly Fire Fire Fire is a giant fireball that spreads spreading flames, so you might not need immutable order.
My personal choice would be to get rid of glyph of dominion.
But this suggestion is mostly related to the fact that I positively hate the node that triggers Glyhp of Dominion on flame rush.
Normal flame rush is one of my favourite movement skills.
Flame rush with the glyph of dominion proccing feels just so awkward to me, I cannot use it.
I use GGR. It’s the one that actually immediately drop 7 ignite stacks and extended duration as well more DoT but I can definitely give this a try. No harm, no foul!
This is so funny to me, because I am the exact opposite. I love the feel of it with the shortened duration and the auto Glyph drop. I will also try this but I have a feeling personally playstyle may win on on this one. 
So in the current incarnation Frost Claw isn’t going to work. It’s doesn’t allow me to make use of mana burst node to spam and keep my ward up as well as getting back mana buy spamming 0 cost spell.
I tried taking everything in FC that reduced mana but discovered that mana cost reduction and -x mana aren’t the best for interactions (probably by design.)
Base Cost with Scepter - 8. Without Scepter - 12.
Gift of Winter only. Reduces to from base of 12 to 2. (-6 for GoW and -4 for scepter.)
With Scepter, GoW and also Spark of Celerity (-60% mana cost reduction) it gains no reduction. Stays at 2.
Do this in reverse order (i.e. Spark of Celerity first) same result.
Not sure how the math works (as mentioned above I’m not much of a numbers guy) but it’s clear the 60% isn’t applied to the base of 12 and THEN the point cost reduction happens. It must be all base costs FIRST (12 - 4 - 6 = 2) but then I can’t figure out where the 60% reduction part comes in because 60% of 2 seems to be 1.2. So maybe it just rounds up instead of to nearest whole number.
Anyway, the only way I can get this to zero, without really re-gearing/speccing stuff to try to figure out a way to get more mana regen/replenish, is to get another scepter and get the -2 to -4 spell mana cost affix like mentioned above.
So I’m going to keep it with Fireball right now while I wait/find/ascension another scepter. Your idea about just having Runic Innovation though got me thinking that I could still do that and spec another skill but after trying several (Elemental Nova being one) it too drastically changes the play style and I’d rather use that idea for a separate build at some point.
There is another minus 3 a little bit to the left. Slower projectiles, more dmg against frozen ennemies and minus 3 mana cost.
This should bring you to 0
12-6-3-4=-1
And if you eventually slam minus mana cost on a firestarter torch you can get the points back.
Edit/PS : the node says “more dmg to frozen ennemies”, not more cold or more hit dmg. If you ever decide to run a unspecced snap freeze on your bar, you can take advantage of the more dmg with your ignites.
No, flat modifiers are applied first then %, as with anything in LE. It’s applying the 60% reduction from 2 to 0.8 then rounding up to 1, so if it’s still showing 2 then that’s a Bug Reports.
Son of a %^&#$^
I looked all over for more mana reduction/pts. even did a search and somehow did not see that.
I’ll file it because it’s definitely saying 2. I thought that was the proper math but then it’s not my forte so figured I didn’t understand the formula.
I agree 1000% with you the highlight/search should be brighter. I want big flashing arrows and an orchestra.
It’s not mana cost reduction the node gives it’s mana cost efficiency. It’s not the same calculation
Should be: New Mana Cost = Old Mana Cost x 100/(100+Mana Efficiency)
Works like CDR
That still works out to be 1.25 in my estimation.
Old 2 mana
60% mana efficiency
2 x (100/(160) = 1.25
So if this is correct math, than it looks like it is rounding up, not down. Is it supposed to round up?
Yes, it always rounds up.
Ah, I didn’t check the node. But yes, I know how that works.
FYI: Unity uses banker’s rounding by default (it rounds to the nearest even number) – this is used almost everywhere in LE UI where numbers are rounded.
Okay this is why math breaks my brain.
Are you saying in order to get an odd number like 1 or 3 you have to land on it exactly? Otherwise you’ll only get evens?
Like 1.01 or 2.99 will both round to 2?
Is this why some of those early +n skills on weapons were making it almost impossible to get the highest range on them?
I’m thinking that he meant the nearest “whole” number instead of the nearest “even” number. Correct @Dammitt?
Not exactly. Banker’s rounding is sometimes referred to as “rounding to even” or “rounding to nearest even”, but in fact it’s just rounding ties to the nearest even number which basically means numbers ending with 0.5.
Mainly this could cause confusion in cases when some value is calculated and then displayed rounded. Usually when you have some rounding you would assume that 1.5 would round to 2 and 2.5 would round to 3, but in case of banker’s rounding they both would round to 2.
So if for example there’s a skill that costs 3 mana and there’s a node / item mod or whatever that increases its cost by 50%, then it’s mana cost would be 4.5. In UI mana cost is displayed as integer, so even if one might think that it will be displayed as 5, in reality you would see 4.
Okay, that makes…‘sense’. 
Next question. So in the very particular case I was seeing with Frost Claw where it was showing me 2. By the actual math calculation it’s 1.25.
This is apparently being rounded - for display - to 2. Does this mean it’s costing 2 mana or on the backend does it know it only costs 1.25 and that is the ACTUAL number being calculated?
No, the rounding is only for display purposes. The game uses the calculated mana cost (or etc) when the skill is used.
Uh, just checked and seems like I chose a wrong stat for giving an example. Mana cost / channel cost in tooltips are rounded via Mathf.Ceil(). Stats in character sheet on the other hand are rounded via Mathf.RoundToInt(), which uses banker’s rounding.
As for the used mana cost, Llama8 is correct, internally it’s just floats, so it uses 1.25.