a 13.5 or 10.5 setup with the RS will be a nice setup i personally run 13.5/rs in my sc10 and a 13.5/rs setup in my girlfriends slash... both are just right for mod class, easy to drive and just enough power although alot of people run 10.5s and 8.5s the leaders are all running 13.5s they are just easier to drive faster.
I've been running a 10.5 in my sc10 and for me its overkill. Most of the time I have the throttle epa set to 80%. I've also run the same setup in my slash before I sold it, and I really liked the 10.5 in the slash. It just requires a smooth throttle finger.