I'm actually struggling installing/downgrading to sdk 2.1 and 11.12 from 12.8 and sdk 2.7 under W7 x64. CGMiner 2.7.5 didn't like the sdk 2.1 and said that no devices was found/usable even though 2 lines before it lists the sdk.
I had some similar problems but after I cleaned my comp out of all ATI-drivers I managed to get it working
-fankih-