  I have a pretty decent machine for gaming, I don't really notice any decline from Avast on my machine, so I never felt a needed for the use of Game Mode.  This is great -- as others have mentioned here the performance impact from Avast is already very low (an important factor for our developers) so I think the most difference would be noticed by either the most hardcore gamers, or the other end of the spectrum, lower spec machines that may benefit more from the changes.  