simulate this circuit – Schematic created using CircuitLab
Figure 1. D1 is reverse biased, no current can flow through it and it will have no effect on the motor voltage or current while running. If the supply is cut then the motor will drive current through D1.
Will placing a diode across a DC motor reduce or even remove the back EMF to allow the motor to achieve higher torque at higher rpm's?
No. It will do nothing.
I know the motor will begin heating.
No. The diode will do nothing. The current, voltage, speed and back EMF will not be affected. As a result there will be no additional heat generation.
However, we'll be using a cooling system to control and keep the temperatures down.