Casting/attack speed and bonus stacking

I found that the new boss challenges are a perfect place to test item stacking. I can have a healer and one hero fight the boss while I time and count attacks/casting.

I've tried this with no bonus items, one bonus item and lots of bonus items. I fight for a set length of time and count the attacks. In the case of Kassia i got 14 shots in 10 seconds with no items, and 16 with a +27% item and 16 with lots of bonus items. Similar result with healing speeds or Z'dar's attack.

I sure wish I had known this. I've been favoring the speed bonuses over magic defense etc. Now I can see I should keep an item with each bonus.

Anyone with different observations?