I've done some testing with my tekin motors and thought the "oversized" rotors took away too much rpm for most tracks. 17.5's, with their lack of rpm, seem to operate best with the 12.3 to 12.5 rotors on all but the smallest tracks. I don't think most places bother to check because there really isn't much of a benefit. Roar just has to make a rule so that there can be no argument about "perceived" advantages.