Yeah, this is a bad idea. First of all, cooling down the parts that much and then heating them up again so quickly will damage them; brake rotors are sometimes cyrogenically treated to harden them, but they have to be cooled and re-warmed over several days to avoid damage. Second, cooling down the surface doesn't do squat to cool down the interior, which is where heat-related damage actually occurs.
If you want to do one speed run after another and your motor can't handle it, you need a better motor for the job. Simple as that.
Check out "Fyrstormer's Garage" in the General RC forum.