I'm running the Cirtix 17.5 combo with good sucess. Temps are at ~140 with 3.85 fdr. Tried a 3.4 fdr and melted the plastics diffs (but a noob mistake) so Im sure the car can handle more but at this FDR, no infield speed at all.
Replaced the diffs with the aluminum and reinforced pieces with great initial testing.
The Cirtix 17.5 combo is the local's race spec so I can't see how the car could handle something faster. I've even seen silver can races...whichever motor/esc, this car is fun.