The only thing that 30% fuel wears out faster is your connecting rod and bearings. Your piston and sleeve is still getting the same exact lubrication regardless of the nitro % (as long as the fuel your using has good quality lubricants) so the piston and sleeve will last just as long on 30% as it will on 20%. There might be a slight difference probably due to the engine reaching higher RPMs but thats about it.

Theres still nothing wrong with 20% nitro. With off-road its more about driver and setup than it is about power.