The use the same bumper on all of the models, so the V6 would have greater two capabilities. The manual should be tailored to the engine, so in this case the 4 cylinder and you would be looking at 2000 lbs. The tongue weight comes into play on the rear suspension as you don't want the rear compressed at the front aiming for the stars. If you position the bikes correctly on the trailer, the tongue weight will be minimailzed.
The bean counters have a say in the ratings. Usually the ratings are under the max of the vehicle.