I pull a gooseneck trailer with mine often and it is always heavy (hence why I use a gooseneck). Because some of the pin weight of the trailer is transferred to the front axle with a gooseneck, the effects of the truck already sitting level is more negated. It is slightly nose high but hardly noticeable.
With the factory rake, the bumper pull trailer results in a level ride when loaded. With a leveling kit the nose will be up higher because the factory rake is removed (by raising the front to make it level with the rear). There is no additional sag with a leveling kit regardless of what style of trailer your using. It may look like that due to the nose up look under load (because the factory rake is removed), but there is no change to the rear springs or the capacity of the suspension as a whole.
