short answer is it will be very difficult to get significantly more torque from the motor without also burning up the windings. i can't tell you off the top of my head what it takes to demagnetize them, but its on the order of 2T minimum, this is achievable in a motor. 3T is probably what it takes and you cannot generate those fields without burning up your motor.. literally the windings would melt in just a few seconds.
if you can get your lead acid batteries warm during discharge, they are not going to last very long.
also the battery terminal voltage will drop under load, significantly as the battery discharges. given that you are discharging them fast enough to get them warm, i would estimate you have 11.5 volts available under full load initially, at 40% capacity remaining, your battery terminal volts are probably around 9.5. this will account for part of your torque drop. also at lower battery terminal volts, the maximum speed of your bike will drop proportinally.
it is possible to run a synchronous (aka outrunner, bldc or brushless) motor above its nominal "speed limit" but you have to recirculate current in the windings to demagnetize the motor. this can be done, up to about double the rpm, but it costs you a lot in terms of copper loss in the motor, and power loss in the controller.
i dunno, perhaps 95% of ESC or E bike motor controllers cannot run the motor faster than the battery voltage at unity power factor. basically spin the motor with a drill, rectify the ac power, measure the dc voltage, that dc voltage can't exceed the battery. --that's where the speed limit comes in.
---so it might be possible to wire in a Wye/Delta switch.. the problem is the phase of the hall effect sensors would be thrown off.
such a winding change can be done with relays and would get your maximum speed up to 1.73 times as fast. if its already delta then you could cut the windings in half and then reconnect for wye, it just gets more complicated but can still be done.