Lower kv and taller gearing = less load on the motor and lower rpm at top speed. A Neu will take rpm upward of 60k but for the least heat and best efficency you'd like to be running around the same rpm's as a nitro, mid to upper 30's. The enticement to run 3s is the weight difference between the 1515 and the 1512 as well as the 4s and the 3s. On 3s you could run a considerably higher mah pack and still have a much lighter car although it will run warmer due to higher amp spikes on the lower voltage. It's a tough call...