A Neu 1512 can also be overkill for
a truggy, as it is able of peaks up to 2.5KW. But gearing the car properly becomes more of an issue with a smaller motor as it will heat up faster then a bigger motor under heavy load.
A neu 1509 is listed with 1KW constant and 2KW peak power. The highest peaks i saw in a heavy buggy (9lbs with 10s1p A123) geared for 40mph+ were between 1500-1900 Watt with an average of 250-400 Watt.
This setup would have been "undriveable" for a track as it was way too agressive. All this let me to believe that a Neu 1509 fits from a power point of view.
The question is for what speeds do want to gear your buggy and at which outside temperatures will you run your buggy? Judging by your signature you already run a buggy with a Neu 1512. What temperatures do you get with this setup on a typical run? If you gear a Neu 1509 for similar speed i would suspect it to run somewhat hotter as it has to draw more amps to give you similar torque to your Neu 1512.