Just had a similar problem with my 5750, hope my solution also works for you. First make absolutely sure that you have the correct device ID (sorry if that goes w/o saying). Putting it in ATI5000Controller.kext should give you full resolution support. Putting it in AMDRadeonX3000.kext should give you QE/CI support. In AMDRadeonX3000 there are sections for various graphics card families or code names. Be sure to put you DevID in the correct section. Your card should go in the AMDCedarGraphicsAccelerator section. (I do not have this card, so I cannot verify that it is Cedar).
Also, I don't think your card is a natively supported card, so you may need GraphicsEnabler=Yes. Try both Yes and No.
I hope there's something here that you haven't tried yet. Good luck.