Fwiw i installed a 250w yose rear wheel kit circa 2 years ago, i also bought a kt dual voltage controller with the intention of sourcing a 2nd 48v battery as i am a larger non enthusiastic cyclist.
2 years on and im still riding with the stock system, the kt controller is installed on a 2nd 'backup'

bike..
limited experience 'test ridding' the kt 17a controller with 36v and no speed limit and i achieved 22mph but the last few miles were slow painfully slow to attain on the flat.
I would suggest buying a 250w kit and if you find the 15.5 mph speed limit too restrictive and you have confidence in your bikes brakes and stopping distance a 17a controller upgrade alongside a battery upgrade to 48v will give you both 33% more torque and speed putting 20 mph clearly at the optimum running speed (15 + 33% = 20)
Though moving at 20 mph in unlikely conditions (anything but down hill) is likely to draw interest and with a bike unrestricted to 15mph, it might not remain in your possession.
And you might want to monitor the motor temp considering the extra amps you will be providing it.