FWIW, I've never seen "AAA" nimh cells that were happy putting out more than about 6 amps. There's no max. efficiency current listed for the motor, but if it has a 15 amp ceiling, I'd bet you're well below the most efficient current level. Personally, if I had to use nimh cells on a motor with those specs, I'd be looking at 8 cells of 2/3AA or 2/3A to start with. They handle quite a bit more current than the AAA cells do, and the 2/3AAs aren't TOO much heavier. After a bit of searching, it appears that motor is very similar to a Feigao 1308416L as listed by Balsa Products. So with the 4.3:1 gearing you have, it appears that 7-8 2/3AA cells like CBP 750 and a GWS 9x5HD or 10x6HD prop might be a good starting point.