Don't know about a 20# truck but I do have a 12# truck that is brushless and pretty bad too.
I don't gear it for speed though. If I did it would do 50 easy. I have a 1/8 buggy that has hit 60 that's basicly the same thing only a couple pounds lighter.
My buggy right now is geared to top out at 44-45mph.. and it can run all day like that.
Your Genesis could be done... but the bigger the vehicle the more $$ it takes to make it go fast on electic.
Motors, escs, batts... all cost $300-400 or more each...
There are a couple guys I know doing 1/5 electrics....