Chances are, your motor can easily handle 1500w. Nearly all 500w rated motors can, and the direct drive ones even more.
So your motor should hack it fine. give it a 22 or 25 amps controller with no worries. But make sure its not too much amps for your battery. Pushing your battery hard will kill it sooner. But you need an excuse to get a new 48v anyway right? Don't go all the way to a 35 amps controller unless you have the cash to back it up with a very strong battery. Usually this means a big one, expensive. At least 20 ah.
More wattage should get a tad more speed, but not much. It will get you to whatever the top speed for a particular voltage faster. It might still top out at 20 mph with 750w of 36v, But I bet you get closer to 23 mph on 36v, once you have the amps. And once you go back to 48v, 1000w might get you to 27-30 mph.