It's a combination of electronics, drivetrain and weight. Most of us take out the stock electronics and run third party control boards like the Clark TK-22 for example. I run the TK-22 on my WSN T-34 and I get very good control at low speeds, this is due to the quality of the electronics and the added momentum feature that eliminates the jerky low speed start. However, part of the reason why the tank runs so smooth is because I'm running a stock chassis: WSN gearbox with plastic gears, plastic tracks and plastic lower hull. Overall my T-34 weights less than my Panzer III believe or not.

I also run the TK-22 on my Panther, which has metal wheels, metal tracks, and a heavier speaker. In this case the low speed performance is not as good as the T-34, due to the added weight of the metal parts and also because I'm running the stock HL pot metal gearbox with stock motors. With heavier tanks the high torque motors and gearboxes are crucial. A popular upgrade are the 4:1 steel gear gearboxes with black 400/480 motors, HL and Taigen both make them with bearings as well.

Adding 540 motors would involve modifying the existing gearbox, I have seen it done as well.