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.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.