I run surround for more than 5 years now, i think i kind of know what it takes to run surround.
I really simple and popular exemple is BF4. SLI GTX 670 or 680 2gb or 4gb SLI will cap out at pretty much the same settings to remain playable, not choppy. ULTRA and 2x MSAA. Sinlge of both card at 5760x1080 wont allow MSAA. ( notice that anandtech are using FXAA at 5760x1080, not MSAA ). I'm sure a Titan can handle 4x MSAA at 5760x1080 while 680 4gb SLI wont, the game become a little choppy. If the 680/70 were 320+bit and 3gb, i'm sure they could have nuked AMD away when it comes to surround.
Surround need more than a good GPU, it also need a very strong memory bus and a good amount of memory to be able to max out recent games. Newer games may need even more. + a single GPU is alwais better than 2 cause you will never ever fall into multiGPU scaling issue in few titles.
EDIT : OP, Wait a little bit, a Titan LE is at the next door. ~650-750$ 2304sp 5gb on 320bit.