Lot of variables here.
First, if you disconnect the Motor leads from the PS-2 board, nothing you do to the motors will hurt the PS-2 board since they are not connected.
BUT, sound effects produced are effected by what the tach reader sees, and possible feedback from the motor leads (not sure on this).
Second, the motors are DC motors. If you are using AC track voltage, you still need to rectify the AC to DC before feeding the motors. Since speed control is now track voltage changes, the PS-2 board will see varying voltage also. Since you are operating conventional, you don't really need the PS-2 board.
Not sure what mods have created an engine that is too slow at max throttle, but the DC motors are rated at 12VDC, so how fast will it go at 12VDC with your gearing? If you have to raise voltage higher, you risk burning out your motors.
Another alternative for you, may be to go to a PS-1 system. You get conventional operation with sounds and you can install a PS-1 smoke unit in the unit too.
I have them available if you need a set up. G