I run a similar setup only mine is Watercooled and stupidly overclocked.. vroom vroom.

In WoW (only game I really mbox) when I played 15+ characters without SLI and running each card independently on a monitor (I only run 1440p, x3 monitors) I would experience a lot of weird slowdowns and what I called 'hiccups' where it would freeze for half a second then catch up. Using that nifty HWinfo program it showed the GPUs (980ti's x3) randomly hitting their limit (core load, memory usage) which caused the stuttering.

Turned on SLI and everything smoothed out and i'm hitting my cap of 60fps with no issues. Saw no benefit of 144fps in a game like WoW haha.. it's not that competitive..
ANYWHO. I'm in Dalaran right now, 15 characters spamming spells.. the area is packed and my GPUs in SLI are showing a 38% memory usage and a core/bus load of under 14%. Which is sweet FA. My advice is to buy ALL THE VIDEO CARDS!