[mjlaupp]

Originally Posted by harveyrv

.....the weight of the tongue of the trailer is lifted off of the trailer hitch by the upward thrust, then falls back down by the force of gravity......

Please explain the details of the source and mechanics of the upward thrust that lifts the tongue off of the hitch. The only source of lift (upward thrust) on the trailer tongue that is capable of lifting the tongue off of the hitch that I know of is from the tongue jack on the ground.

.....This over-loading and chance of bottoming out will be more severe with 1200# spring bars than with 600# spring bars......

You have this backwards. You need a stronger spring to prevent bottoming out. This applies to tow vehicle axles, trailer axles and weight distributing hitches. However, you can overload the hitch and/or tongue with excessive rated spring bars.