The inductance of that motor must be pretty low relative to its resistance. Assuming a DCΩ of 24V / 8A = 3Ω, the inductance would have to be in the range of 50 or 100-ish µH. I'm accustomed to PWM motor control implying frequencies above the audio range. At 16 or 20KHz and a millihenry with a flywheel, my DC and rms values don't differ by enough to worry me much. At 2KHz, the motors I work with would emit a very annoying midrange whine and some frame members and covers tend to resonate around 2KHz as well.
Use the rms current to calculate power being dissipated by the motor. Unless there are cooling fans involved, the heating of the motor will be proportional to I * R squared where R is the DC resistance of the motor. Lowering the voltage will not allow you to increase the amperage. Lowering the voltage merely lowers the rating. 30W at 24V implies 15W at 12V. You're going to be stuck at 1.25 Amps no matter what. At 30W, I would also assume the rating is for power consumed and not power delivered. 30W input at 80% efficiency = 24W output.