The replica bodies would be nice, but if they're Pede sized I'm pretty sure they'll be too small on a GP or Clod. Someone with a Pede can confirm though. If they made Maxx sized one, that would be good for a lot of people even if the bodies aren't perfect replicas.
Traxxas involvement with Monster Jam will only help the Ground Pounder, Wheely King, and Clod sales and overall awareness. A 2WD/2WS Stampede will sell, but people will also see the more realistic 4WD/4WS solid axle platforms that are available.