Donkey has it right.
Reducing the power on the downwing engine in a level turn would do a couple of things. At the beginning of the turn (i.e., as you roll into the turn) it would provide a very slight improvement in roll rate into the turn which you would probably not even notice (this is caused by yaw and dihedral effect). Is it worth it? No. Use your rudder.
Reduced power in the turn means you will reduce your sustained turn rate (again, in a level turn). In a level turn the only thing fighting drag is your engine power, reduce it and drag becomes more dominant and you will slow down (lose E) at a greater rate. Also, split throttles would mean an uncoordinated turn which causes even more drag unless you can keep the ball centered in the turn without rudder.
Reducing power on the downwing engine in a vertical rudder reversal should help as you're trying to swing your nose down at slow speed where your flight controls have reduced power so you're using thrust to augment the yaw rate. That's provided of course that the differential thrust doesn't cause you to depart.
The amount of torque produced is related to MP, not RPM. MP is a direct indication of how much power you are applying through the prop. Low MP = low power = reduced torque/p-factor. RPM does relate to gyroscopic effects (the effect of a rotating mass) which has some effect at very low airspeeds (and in something called a coupled departure which you won't see here in AH) but it's small compared to the torque.
