Clearly any game would basically have to assume 90% is the minimum it could get and not absolutely rely on more. It's possible a game using this could use dynamic resolution or something, or just use the extra power to not drop frames as badly, but don't expect higher quality graphics, since games wll never be able to assume the power is always around unless ms somehow feels like disabling snap for one game (which is a bad idea).
And yes, this is somewhat of a desperate move; games traditionally on consoles assume fixed resources and are faster because of it; it's entirely uncertain how well a game could both do this and also use the extra resources it may have for a performance boost in a non-negligible way without complicated programming that most games will never bother with.
So assuming this is true it could make development notably more complex if developers try to take advantage of the additional allocation?