The key is in the numbers, I'm having the same issue. If your asking if you can run 1500watts you have to name the components. The original controller
States 50 amp 84V = 4200 watts. The math doesn't add up there's no way your feeding that kind of power with that controller. So the question or information is this, 1. How much power do you have, the stock system is 72v 20 Ah. = 1440 watts. So are you using a different pack and what's the output power available. 2. what controller are you using, the stock claims amazing specs but to be legal it's restricting the power to the rear wheel to???, personally I believe it's a 25amp controller restricting the power to 500 watts. But the controller supplies the rear wheel with power. 3. What hub motor, 500w is the legal maximum. So what happens if you feed it more, it goes faster, and faster, and faster and hotter and hotter until it reaches saturation and stops going for ever.
So back to the main point, what are you using, I'm personally going to get a Lyen or Kelly controller, feed it with 52 volts 30Ah lithium and if the motor won't spin up enough get a new motor rated for the lower voltage of my pack.
I could simply run the motor backwards, set it to run in reverse and take off the limit as there's no upper regulation to run backwards, only forwards, this theoretically gives me 90 + km/hr using stock components. But 52v battery's are cheaper to build and lighter to carry.
Maybe if you state what exact components you are using and at what voltage it would be easier to give a clear answer. The stock hub will probably take more power for sure. The controller seems to be the big unknown and the numbers are probably no more than marketing hype, as soon as my CA turns up I can give numbers.