CG is not designed to create miracles, it just makes programming easier.
If CG is to be adopted, it will be because of its ease of use, cross platform capabilities and code optimizing for diferent cards (if manufacturers other than nvidia support it)
Microsoft is not known to be good for optimizing software, only to get the job done, so nvidia could optimize cg to perform at least as fast as HLSH. (there's still no definite proof that CG is actually slower than HLSL anyway)
Sometimes I hate being right everytime.