I recently upgraded my Dell M17xR3 to Windows 8. After install, I noticed that my screen brightness had been reduced to almost half of the original brightness, although brightness control seems to be at 100%. I have disabled auto brightness, and I have tried several different video card drivers (microsoft's own, AMD mobility latest, Dell's recommended AMD driver). I also installed the latest version of Alienware tools. My Dell has an AMD HD 6870 card.
Using different drivers, I also noticed that the brightness slider would not change the brightness at all steps - with one driver it would not do anything during the first three steps, with another it would alter brightness once every two steps.
The bottom line is that my "max" brightness is not really max - it's about 50-60% of the max brightness that I had before.