You can try your bike set up with a b(o)rrowed 52v battery. In theory you should get 8% more watts and your motor will turn faster. I don't know if it will be enough for 4 additional mph.
A truly smart charger should control the battery cells individually, charging time and total voltage is not a guarantee of a good balanced charge. The way the battery cells are soldered to the battery monitor system dictates the charge process. My advice would be to trust your BMS and charger...
64 gr. , I know I have one and it's the last thing I would bring on a ride. I bought one for drones and falconry and never used it. My own perception of the wind is accurate enough.
I don't think you should expect a lot better from this battery. Your controller can draw, wide open throttle, 52.4V x 18AH = 936 Watts/hour and your full battery 524 Watts/hour, that is less than 1/2 hour of wide open throttle before the BMS cut.