I did some research. ATI does have
specific drivers for usage under Boot Camp but are much older than 11.5.
Another idea was to find the Device ID of the device from the .inf files in the ATI folder the installer creates on the C: drive. You would have to go through the Windows driver installation dialog and effectively force Windows to install the drivers from for the proper device from the .inf list.
Someone else is looking for an answer for their iMac as well.
You might want to install the included drivers and upgrade as well if you have not tried that yet. I can understand the desire for a clean driver install.