Stick with 5 or 6s. I have found that my 4500 30c 6s lipos are close to maxed out with the hacker 11xl. I am not sure if the 1521 will draw less amps, really do not think it will. I would stick with the 4900 5s pack and a lower kv motor.
Ok, what about 2x3s? NEU Energy 4900mAh 3s 25-50C? And a NEU 1521 with 1900Kv?
That would be good I think?