Monitor cpu and gpu usage, if the cpu usage is 100% while gaming and the gpu's are less, maybe 50% ish then there is a good chance the cpu is bottlenecking the gpu's, i would say tho a 920 @ 4.2GHz is more than enough for thoes cards, try overclocking the gpu's a bit and test, if u get more fps then it's probably the gpu's.
Thanks for the tip.