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.
For some reason, standalone 2.1 SDK doesn't have certain things present to allow miner software to detect it. You need another SDK installed along side it (2.4 or higher). Use the "gpu-platform" command to select the appropriate 2.1 sdk platform once you have both sdk installed