I believe, this may not be true, that when a motor is available for two voltages like 24 and 36, the lower voltage model will spin faster at 36V but have less torque than the 36V motor on 36V. So you could go faster if the motor is strong enough.

On the other hand, you're saying your current kit is good for 16 mph at 36V. Is there any electronic speed limiter going on? If not, that's pretty weak. My opinion is that your 36V motor and controller hasn't got enough power. Most 36V kits that I've seen will go 18-20 mph with 26" wheels.

You should ask Golden Motor.