Just to pick up on this point, it's really a matter of current.
Depending on the design of the motor, its power is related to the strength of permanent magnets designed into the motor, the winding(s) - (thickness of wire and number of turns etc), how any electronics driving it have been designed etc, and ultimately the amount of current you can put through the system before it starts overheating and destroying itself.
The only way you have to increase motor power "from the outside" is to increase the voltage, which seems what you are motivated to do. But it risks burning out the motor, and anything else in the tool that uses that voltage.
If the motor were to short circuit, the amount of energy in the lithium battery, liberated all at once, can cause a fire or explosion. That's my main concern - your safety!
If you want a more powerful hedge cutter, you need one with a more powerful motor.
Edit - just been looking at the Ferrex 20/40V battery. You
can fit it to your hedge trimmer, but the trimmer will still run from the 20V connection, with the same result as at present. The battery may last a bit longer than the ordinary 20V one though.