BootCamp is only supported by Apple, so they will be the only ones who can provide a working BootCamp driver for your system. The only mobility drivers offered by AMD are reference drivers, generic ones that aren't guaranteed to work due to changes made by the laptop manufacturer. Since your card will have been flashed with an Apple VBIOS by Apple, they will unfortunately be the only source for drivers for an Apple laptop with a Radeon card, whether in OSX or BootCamp.
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