I have a XFX 8400GS 256Mb id 0x06e4 working fine on games and everything else with the VGA port, as I don't have a DVI monitor. Right now I'm using EFi string but I've tried every injector out there and they all work fine at stock config, no edit needed.
What you may try is to create a custom EFI string and see what comes out.
To do that I've used OSX86Tools but I think EFIStudio should work too. Just make sure to select VGA/DVI-I as your output.
Hope it helps.