I've 2009 Mac Pro with 2xNehalem 2.66GHz processors. I've just tested GTX 660 (even EFI-flashed), GTX 970 and GTX 750Ti. What I've found out they work at nearly 2/3 of what they could be. I've compared with PC results of Unigine Heaven and Valley test and get that it is so. How can we get GTX-card works for the full power? May be the trouble is in processor and upgrade 4,1->5,1 with 2x5650 Xeon will make the deal?