In this case bigger is not better as at these power levels your batteries are the limiting factor. I would go with the 1515 1Y. This is the motor that castle creations recommends for the MMM and 6s and they claim 60 mph. The 1Y has just a bit more rpm than the 2D, and Y winds are slightly more efficient than D winds.
Check out my custom converted 8ight:
Sub 7lb, lowest CG of any 1/8 buggy