MBP Boot Camp Windows 8 brightness issue
It appears that there is an Adaptive Brightness feature in Windows 8 that messes with the overall brightness of the MBP

If you just installed Windows 8 on your laptop and the screen won’t
stay at the brightness level you want, it’s probably because the
adaptive brightness feature isn’t working right on your system. Here’s
how to disable it.


Open up the Windows desktop with Win + D, click the power management
icon in the tray, and then choose More power options from the menu.
Once here, click the Change plan settings link on the selected
plan.
Then click the Change advanced power settings link.
And now, browse down to Display &gt; Enable adaptive brightness, and
change the settings there to Off.


Naturally, it would be best to have the feature actually work, so you
could make sure to update your drivers. If you still want to disable,
here’s how to do it.
Just so we’re clear, here’s how to change the brightness on your
Windows 8 laptop… hit the Win+I shortcut key combination (that’s a
capital i), and then you’ll see the brightness control right there.

Disable Windows 8 Adaptive Brightness