I found that My MSI Nvidia GTX 550 Ti can work at 10.9.5 and 10.10.5, but not working at 10.11 or 10.11.1.
The key issues are :
1. Need set GraphicsEnabler=Yes and nvda_drv=1
2. Must use smbios=iMac 14, 2 version from Chameleon Wizard
3. Booting with nv_disable=1 to login Desktop, then install Web Nvidia Driver :