what happens if you uninstall the drivers again? does it speed back up?
they are both AMD (Radeon) cards - so you would not expect difficulties..
were you using the stock AMd drivers? or special ones from Sapphire/HIS?
Assuming it speeds up when you uninstall....
I would install the stock AMD ones from here http://support.amd.com/us/gpudownload/Pages/index.aspx
If that still gives you grief... unisnatll.. use the AMd uninstall tool to clean it up.. and try again.