My opinion with the 1521 vs 1515 is that the 1515 size already has more than enough torque to deal with the last thing you need is more.....remember torque is a twisting motion and it will be harder on your drivetrain compenents. With a 1515/1y (2200 KV) it is pretty out of control with 5s, you will would spend a lot of effort trying to make a 6s version drivable