My kids are currently playing the "Grip Shift" challenge levels demo. I'm looking at it, and a few things become apparent:

1) The art is good, but it's actually fairly basic. Each level is small and self-contained, and they nicely reduce the need for environmental art by making them float in the air.

2) The game physics is fairly basic -- the car can "glue" itself to the road when it's upside down.

3) This game does not respect title safe zones on HD displays for various instructional text!

All in all, I believe this title could totally be written in XNA GS. I think the single-player game (the challenge levels) would be a pretty good bar for anyone trying to make an independent game and compete "for real" with XNA GS.