"BEST PERFORMANCE" is more than just getting the best graphics cards.
For example, I'd rather have a single GTX970 and a 27" 1440p G-Sync monitor (like the Asus SWIFT) then have 2xGTX980 but only a 1080p non-GSYNC monitor.
So it's smart to think about the entire system.
It's worth noting also that 2xGTX980Ti is arguably massive overkill. Would you get a better experience than someone else?
For example, let's say you ran a game at 4K resolution. You'll get about 55% the frame rate as you would at 1440p resolution but it probably looks identical. So you might have to buy a second GTX980Ti to maintain the desire frame rate but essentially it's a waste as it might not look any different.
If I was building a top-end system today I'd...