a lot of times you will lose top end with a .21 because they normally (less than 300 bucks) do not have the rpm range of a .15. And a 2.5 is just the .15's displacement in CC's instead of cubic inches. Just a marketing plow. Like TMAXX medic said, it is all in the gearing and a tmaxx also has the aerodynamics of a brick wall so it would not be easy either.