Other than the voltage drop through the full wave rectifier on board, and the voltage drop in the output transistors, decoders do not produce voltage lower than the voltage on the rails.
Since the minimum voltage the decoder is specced at is 7 volts, that's the minimum voltage you will send to the motor.
All of these decoders use PWM, on off switching of full track voltage (subject to the caveats in the first paragraph)
All that said, PWM can lower the AVERAGE voltage, and as long as you don't overheat the motor or otherwise break down the insulation on the motor windings, you are ok.
Greg