I don't think 6s would be a good idea. I believe the over-voltage system of the Velineon is both hardware and software protected. As of yet I don't know of anyone who has been able to run more than 3s on it. Also I don't think you want 6s with that motor. People say it gets hot already, and with double the power going into it I can only imagine how it would react. I think the FET's in the Velineon are rated at 25V which means the system probably couldn't handle 6s anyways even if you could bypass the over-voltage system.
I think you should Email Traxxas and tell them you want to Beta-test the E-Maxx VXL.
