No matter how powerful the Wii U is going to be, 1080p will be very hard considering that there's also at least one Upad taking up a considerable part of the fillrate. Most developers are probably going to optimize for 720p rather than fiddle around to get stuff running at 1080p.http://www.neogaf.com/forum/showpost.php?p=37822903&postcount=8417
I would say 5%
My info was relevant for the v4 dev kit.
The following one bring less than 1y0% increased performances, so it wouldn't be enough to manage a good looking current HD platforms game at 1080p. Maybe with optimizations from their engines, the SDK, the middleware used, etc, they can fiddle a few resources, but i think it won't be enough, considering the framerate wasn't 60fps at 720p for their title. Of course, it's on a third-parties, "port", and intricate use of the padlet, context.
The tradeoff is probably too big, and few people are going to notice the game running in 1080p anyway.
I just hope Smash and Pikmin are going to run in full HD. Resolution would actually add something to those games.