The vxl speed control is only rated to 3C, based on that, I don't know what you can get that would run 3C well? there might be a fiego (they run hot), could even try a Novak 6.5 (if it will work with your speed control).
Essentiall, I can't answer this question properly, but I tried anyhow. Have you tried searching the forum? there have been some posts on the traxxas brushless systems, might be something useful.