Cool beans.
As Im sure you already know, other than diameter changing. Foams (especially thinner side walls) wont absorb as much as the small imperfections of the surface it rides on. So your suspension might have to be slightly softer to compensate versus running rubbers. Foam shores effect this as well with the softer shores allowing a bit more absorption than the stiffer.
Just so you can get an idea of how small they run foams here, I have included a picture showing Zack Barrys three cars from the Nats warmup race. As you can get a better sense of tire diameters between them. This was at a carpet only "off-road" track, so the various surfaces you run on at once for the UK club meets may not apply.
From L to R
M18T w/Xray Foams. M18MT with Rubbers (Rules). M18"B" with Rubbers (Rules)