Hello! I got it, the 660Ti 2048MB non Apple. Before on a 3,1 now on 5,1. Works.

On 10.9.4 seems works a lot of GTX, also the 690, but don’t know in 3,1, if power supply is enough.
Yo don’t have the EFI boot but as I learn around the web, if you need it you should leave en Apple for screen and use the GTX just for CUDA.
Apple Mac Pro “Twelve Core” 2.93 (2010/Westmere)
NVIDIA GeForce GTX 660 Ti 2048 MB
48GB RAM DDR3 1333
CalDigit USBxHCI
Intensity Pro