it saddle pack just might fit, but it would be illegal. in general the class races with a sensored motor and 4 cell (4.8v nominal) which is getting replaced very quickly by 1s lipo (3.7v nominal)
i dont think we care much about the class racing.. a bunch of us are just getting into it. 1c with the booster seems like a pain
I dont think the 12R5.1 needs a saddle pack, or would most 2S be too tall to fit under the shock.