Bigger isn't always better. The 21's will rev higher. The 28 will have more torque, which will accomplish little more than destroying your drivetrain faster. Get an Argus A52 or the new B52 which is supposedly even more powerful.., Werks, OS, Novarossi, Go GX II, RB Fire 11, etc...

If it has to be bigger and overpowered, than an OS 25XZ. But you really might as well spend now on something you can at least race later if you want, but that will still be really fast...