Here's an easy fix.  In Quake4 for 360, 480i/480p seems to have a lot less slowdown than 720p/1080i.  I've heard that Project Gotham Racing 3 in 720p/1080i is actually being rendered internally at 1024x600 to fit into the 10mb of edram then scaled up to 720p/1080i, as 1280x720 with 2xAA is too large to fit into the 10mb of edram without using predicated tiling.

How about releasing a patch that downgrades the 1280x720 internal rendering of the game to 1024x600... I would think making this adjustment wouldn't be much more difficult than altering the value of a few variables.  Might not look as sharp, but at least the frame will fit in edram and therefore the performance hit likely won't be as big with 720p/1080i w/2xAA as it is now (the slideshow we have now in high def is painful).  While devs in the future should strive to use predicated tiling, in this case downgrading the res a bit would be a decent fix; I couldn't see MS having a problem with it since their own game does the same thing.  If its not already disabled (and it doesn't appear to be), disabling vsync on top of this could also make the difference between the disappointment we have now and a better game; MS' own Perfect Dark Zero appears to have vsync disabled so again I couldn't see MS having a problem with it.