So long as the speeds are set the same as the PC ROM then it should perform OK so long as there is no other significant hardware or software bottleneck.
I have no problems with the flashed cards. Everything, including Sleep, seems to work. With G4 Macs running Leopard there is a boot delay (about 2 minutes) which has to do with the GeForce 6200's temperature sensor. If you want a faster boot you can disable the temp sensor in the card's ROM but I like to see the GPU temp.