For an idea of the speed of boosted foam, at the Halloween Classic this past weekend, the 17.5 foam class had faster lap times (with good amateur drivers) than Paul Lemieux and Keven Hebert did in Mod Rubber. Obviously, the Mod cars were faster down the straight, but the foam cars carried more corner speed due to better traction, resulting in faster lap times. I agree that 17.5 boosted would be faster than 13.5 unboosted.
Unless you've got really good drivers, you'd be better off with unboosted 13.5.
*** The Gate - Celebrating 7 years at the same location ***
The difference between genius and stupidity is that genius has its limits. - Albert Einstein
Understeer is when you hit the wall with the front of the car and oversteer is when you hit the wall with the rear of the car