Here's my thinking:
Torque is rotational force. To realize the full effect, the object acted on needs to resist, push back, absorb etc.
If at 600#ft you blow a ujoint or axle shaft... or the tires just start peeling out, you can't really build much torque to throw down. Nothing to 'push' against.
That's where the weight comes in. If that load prevents you from losing traction (and you don't break things) now you can build torque.
IMO, the clutch rating is the rating. It doesn't change based on the weight of the rig.