I just upgraded my Dell Mobile Precision M6600 with a M6100 Fire Pro to Windows 10. I lost the ability to adjust screen brightness with the function keys, or in the power options screen. I do get the OSD display, but the brightness isn't actually changed.
I have tried several drivers from both the AMD and DELL sites, all of which misreported the card as a M8950 until I finally found a special version for DELL on the AMD site which identifies it correctly (version 15.201.2401.0). None of those drivers helped.