I just bought the expansion pass today (I have a little bit of money for a change) and thought I would have a look through the options only to notice this new option in user.settings. Hopes were raised; after all, this bothered me enough to create this post after the initial release:

http://www.gog.com/forum/the_witcher_3_wild_hunt/witcher_3_another_game_that_ignores_colour_management_rant/page1

Thank you!!!, no more border less window gaming. Although the performance was almost as good as full screen mode, the alt-tabbing was a bit of a pain, so this is very much awesome in my books!

EDIT:
Just realized the option can be accessed easily under "Graphics" option. A little to excited about a little feature that most people don't care about I guess :)