What kind of racing?? For a buggy, I would be more concerned with run times and amp draw. If you can run a smaller motor and achieve the speed you want...as long as its within reason, the amp draw will be less. Mains are sometimes stupid long.
Well, the racing i do is more for fun than it is anything, i'm agreeing that 80mm is most likely too much, but i don't see myself going for anything less than the 70mm.