I have a plastic tranny and i have gone through one set of gears, he plastic will not hold up at all with a direct drive. the clutch setup may help some but theses gears are not meant for that amount of torque.
Good grease goes a long way to make the plastic gears last. I like tamiya's ceramic grease for the plastic gears and the anti-wear for metal gears. I have run a few revos both 2 speed and locked and the plastic stuff has held up for me. I do shim everything though.