I'm using tri sli 570's and I agree that it's not that much of an improvement to justify spending money. There is no real difference in using 3 cards over 2 card. slightly more fps for the price is not worth it. Also can you really see any difference from 60fps-200fps, no not really to most people. I'm running a 3 year old set up : i7 990x @ 4.3ghz ,Asus rampage III extreme ,12g Kingston hyperX 1600mhz @ 1920mhz, 3 evga gtx 570's @ 850mhz, and only get 20-30fps more than using just 2 way sli. not much of an improvement for the cost. I fooled around and used a gts 450 for a dedicated PhysX card and 2way 570 sli and got the same results in fps when benchmarking.
Also having a good fast response time monitor will help out with better gameplay than using tri sli. A good monitor is key to keeping up with the gpu output.
The only reason I have tri sli is because I have 3 gtx 570's and want wanted to put them to use than just sitting on a shelf. The 570's are still a very good gpu and can play any game out there still. Maybe in a few years I will build another gaming system when this one I have now gives out .