The 1500-2200 k/v is in a better ballpark for the voltage you're talking about in the typical type of trucks and range of gearing we run.
The e-maxx tranny in the G-revo and maxx has quite a bit of reduction compared to a typical 1/8 buggy set up where the only reduction in the center tranny is the pinion to spur.
Under 20,000 rpm could be done with the e-maxx tranny but not with the pinions we have commonly available.
A truggy with a center diff would be easier though to get 35-40 mph from 20,000 rpm.Some of the guys here have excel spreadsheets for gearing that would help.
This is Serums,i haven't seen it though 'cause i haven't got excel:
http://www.serum.demon.nl/speedsavage.xls