Ben - that all makes perfect sense to me and what I was thinking about when looking the design over after getting my scale numbers. There just isnt enough leverage for the amount of force the chains are applying. It works to a point and then rapidly diminishing returns start happening.
BTW this is the verbatim quote I got from Andersen when I asked about the pressure put on the rear of the coupler - "The marring of the ball and the pressure applied to the coupler does not concern us as the force is much less than a trailer that is being towed conventionally and every time the brakes are applied the coupler is slammed to a stop on the ball."
I didnt reply to that one as that was also in their reply about something else, but I dont agree with that statement at all. I have never felt my trailer "slamming" into the ball and have never seen wear on the rear of the ball like I do with the Andersen. I have some pretty good and deep wear marks on the ball. I think if your TT brakes are setup correctly there should never be "slamming" into the ball.