Dont you have poor performance with GTX 660 on Yosemite? My 660ti feels weird on Yosemite with UI lag.My Nvidia GTX660 working on 10.10.5 without drivers update
Tip: Don't use the iMac15,1 and Mac Pro 4,1/5.1/6.1 System Definitions. These SysDefs have been found to cause problems. For Haswell builds, toleda recommends iMac 14,1 for HD4600 and iMac14,2 for graphics cards; MacPro6,1 is only appropriate for X99 systems.