I have tried out your fix. While it did give me full screen my performance gain was very minimal, if non existant. I attribute that to driver support though. But monitoring through Afterburner it did show both GPU's firing and at good percentages as well. Around 80%-90% each. But as I said the performance gain was almost non-existant due to driver support. I have not tried different xfire profiles though.
I am running 2 x XFX Radeon HD 6950's, with an I5 3570 and 8GB RAM, all at stock speeds.
I appretiate the effort and am hoping for some driver support from AMD!
If you create a custom profile in Catalyst (to force enable Crossfire) are you selecting the vanilla .exe or the custom one (esofullscreen.exe)? This may be the issue to those who aren't seeing positive results.