It can be a lot worse than that with the wrong CPU, the 8350 only averages 47.9 fps at high settings, so could actually bottleneck a 550 Ti.Starcraft 2 is pretty CPU bottlenecked.
If you look at Tomshardware 1080p benchmarks:
GTX 580: 104.6 fps
GTX Titan: 104.22 fps
GTX 680: 103.97 fps
GTX 670: 103.92 fps
GTX 550Ti: 103.54 fps
GTX 660Ti: 102.93 fps
Maybe you should turn off SLI, then a single 680 can boost faster and you should have better framerate, I don't think SLI works in SC2.for sc2 "only". all you need is a 550TI. put the rest of your money into the fastest cpu you can buy.
run a i7-3770k with 680sli and in large heated battles. frame dip to the low 20. cpu usage is at a pathetic 45% (basically maxed out on 2 core) and gpu is 35% on each (extreme setting with AA).
sc2 is a piss poor optimized game (think blizzard intended it to be that way).