I have not been able to download 11.11 for my Win 7 64-bit notebook, as the link seems to be fishy (which I indicated on General Forums), but as for reverting drivers...
on Windows it is very hard to revert drivers, because they leave a lot of garbage after themselves. The Catalyst Install Manager could be improved on that part. I suggest you do a system restore to the point where you installed the new driver.
Whoops...it seems this massive slowdown was caused by a bug in my code...!
Strangely on Catalyst 11.5-11.10 it never caused any problems whatsoever...suddenly upgrading revealed this bug. I think I may have been writing outside the bounds of a global buffer...but the code worked perfectly with no strange results...
strange? But my fault it was. Hail AMD!