I don’t think this has anything to do with Foes build as a Tester. I heard rumours that also non testers can easily spec into the nodes @Llama8 mentioned. 
Personally I can’t judge the changes in comparison to the old focus because I had not played a sorcerer recently before the changes. I just remembered that focus felt kinda clunky.
But I leveled a fire sorc with the new patch and specced into the nodes that increase mana gain for that 1 second channel time kick. Also took some mana gain nodes on the right side after maxing the other nodes.
I’m playing with volcanic orb and fireball. It doesn’t feel bad in any kind to me. In most cases I don’t have any additional time to focus longer that that 1 second before I have to move again.
It’s a very mobile playstyle even with that one channeling ability.
So for a 400 mana bomber the changes may seem a bit harsh. You guys got used to it and it feels bad now. Would be nice to hear opinions from a new player perspective that isn’t biased by the behaviour of a previous build.
There was also a lot of feedback that focus was that one clunky but mandatory skill that every mage had to use. These changes may be the first step to correct this. Nerfing focus makes using it less rewarding. So you have to look for additional sources of mana control (mana efficiency and mana reg for example).
Maybe there need to be additional mechanics for sorcerers to gain mana back within skills and passives. The Spellblade got a mana on hit passive with the recent updated passive tree.
There’s also discussions about mana in general.