My personal estimate is that 1500w continuous will not likely harm a 9c motor under "normal" riding conditions. I think the return from more watts added does taper off by 2000w. The motor just starts to make heat out of the increased watts past a certain point.
I run a 72v 20 amp controller on my dirt bike all summer, and the riding is far from normal. 1500w is seen on a CA. In winter, when the motor will cool much quicker, I run a 3000w 72v 40 amp controller, but real world, the CA rarely shows more than 2500w for very long.
72v 30 amps controller would be a good compromise, assuming your summer climate is not as brutal as mine.
On the battery side, 72v 10 ah is about 750 watthours. So that means getting wh/mi down to 21. You won't likely see wh/mi that low at 20 mph or above on an MTB. I have gotten 35 miles from a battery that size, but it took riding 15 mph or less.
Typical wh/mi on my dirt bike riding hard is 45-65 wh/mi. 8 miles is my typical ride distance, leaving the batts less than 100% discharged.
So as usual, just cut the range claims in half. SOP to assume any range claim was done at 12 mph.