I looked at the 1940 thread and it seems to me that the 1940/10 might have been undergeared at 13/44. In anycase, whether undergeared or not, I'm not sure that I follow with the overdoing it logic.

If you go with a larger motor, you may have a problem putting a proper load on it. I know that there are some running the 1521 in their trucks, but I don't know that they are doing so in a racing environment. Racing is a grueling test of RC equipment and problems that don't manifest themselves during bash sessions, become readily apparent during or after a race.

I believe that going with the right equipment is better than going with more than you need. I'd say, go with the 1515.