what's the difference?

[lincpimp]

I have a 1521 1y that has m3 threads and a 25mm spacing. I also have another 1521 1y that has m4 threads, but still has the 25mm spacing. A 9s setup should be more efficient than the 6s, but there is no guarantee.

Might also be close to maxed out on the motor adjustment on the mount too. A 1521 will pull more gear than the 1515, that is for sure.