To compare the stats for a particular ability using your current items and a test item from your inventory, merely drag (and hold don’t drop) the item you want to test over the ability to show the ability’s stats as if you have equipped the item. This would be a lot easier than swapping out the item and then swapping it back. Just let me hover the selected item over the ability to see the stats as if I did equip the item.
Heck, go for it all! If I am holding an item and I hover over any of my selected abilities why not show the stats for ALL of them if the currently held item was equipped? Might be a wall of stats though. But it would make it eaiser…