[JThiessen]

I've run two 4.5's, one in an E-maxx and one in a converted Revo. Both worked great. Most people that have overheating issues have it geared wrong (yes, I said MOST, not all). Yes, gear options are "limited", BUT why would you need to change it from the optimum gearing?
To me, the only down side to the HV is the limit of 4S. But you stated that you are shooting for a 40MPH top speed, which is right in the HV 4.5's range, and maybe the 5.5's.
I currently run a MMM/Neu 1515 2.5d, and on 6S it is really too fast for anything but showing off. I'd gear it down, but I'm physically out of room on the 3906 tranny/motor mount.