My i7 3.4GHz, 16GB RAM W764 system had lower FPS on 64 bit client by a significant amount.
Around 30fps in 64 bit mode, 56fps in 32 bit.
This was at a specific location in SW at the same time - doing a spin (i.e. loaded data).
... seems like the problem was that mentioned right above me. Set to 64 bit, then exit client then enter clienter again and I'm at the same performance 64 bit as 32 bit.