Im using a GTX 470 stock speeds and with the Low quality render on the slaves and main on low-med settings i still get 45-50 fps on main costantly. How do you have your cores setup cause that was a huge jump in perfomance for me mine are setup as such.

Main char- Core 1 and 2
1st slave- Core 3 and 4
2nd slave-Core 5 and 6
3rd slave- Core 7 and 8
4th slave- Core 5 and 7

try it out and let me know what happens!