which kind of board-id are you using ?
Mac-F2238BAE which should match imac11,3 which is what I'm using for my SMBIOS config.
UPDATE: So I just deleted a kext from /System/Library/Extensions then rebooted and I was back to getting the dreaded black screen. I'm assuming the values entered in to fakesmc were never being used and I was in fact using an old kext cache with the modified AGDP. I can't remember if I regenerated the kext cache after restoring the original AGPD. I edited AGPD directly and now I'm back to functioning graphics.