As explained in one of the previous speculation thread, third-parties that received the mass production dev kit (roughly 1 month before E3) saw an increase in the framerate of their games instantly, and it seems without having to work hard to adapt their build. I think it happened like that, because my sources got the new boxes, and were able to do some benchmarks with their games and witness the overall increase in performance in percentage (less than 10%) very quickly.
And they burned their E3 demos mere days before the show, so they had 1 month, a tad less, to adapt their content to this new dev kit. It seems, grain of salt, that even with the delay in their shipping, it didn't bring drastic changes (so more like tweaking or minor changes), like a replacement of component, that would have pushed them to re-do a lot of code, adjust their game to this hypothetical new hardware setting in a rush before E3. Or there's also the scenario where there has been an hardware modification, but slight, or done in a way any hardware that it didn't represent an hassle for developers because no one reported me problems with those.
All in all, the games at launch will see a lot of improvements, especially in the image quality department, there will be anti-aliasing (for the ones that lacked it), texture filtering, less problems with texture pop-ups, etc. Thanks to the familiarization of studios with those latest dev kits, new SDK releases, optimizations on their end (their engine, etc.), of middleware used, the fact that they will grow more accustomed with the asymmetric screen setup, and a lot of other parameters, the Wii U titles on the shelves will be prettier than the E3 demos.
Now, my sources told me than any additional enhancement they will retrieve from all these factors will benefit their games in a particular way. Concretely, if they gain 5 frame per second, their builds won't run at 35fps, they will take advantage of those to add more effects, etc, and the framerate will be lowered to 30 fps.
With that said, it must be perfectly clear however that for this context (third-parties projects, most of them being games not made for Wii U from the start, etc.), all these improvements won't make that a game you saw running at 720p/30fps at E3 will be either 1080p/30fps or 720p/60fps. The jump to reach those next visual steps is too big compare to the additional performances they will manage to grab from all the previously mentioned points.