I have a similar problem.
Eyefinity resolution 5228x1050.
2 x AMD HD 6850.
12.1a AMD drivers.
The rest of the system specs are in my profile.
CrossFireX isn't working, so my system is struggling with only one 6850 and three monitors. But even then, with the graphics settings I have to use the resulting graphics quality is similar to rFactor1, only the performance in rFactor2 is worse.
CrossFireX works correctly in my system with rFactor1, iRacing, netKar Pro...
With rFactor2 and CrossFireX enabled, if I run the game in full-screen, the whole system hangs before loading the main menu and I have to make a hard reboot. If I run the game in windowed mode, the secondary card isn't used at all, checked with MSI afterburner.