Brakes and transmission are the "sacrificial items" when you tow over capacity.

Trailer brakes will save your rig brakes, but if they fail, you're in deep s.
Tranny will live longer with huge coolers, and never converters on computer trannys are better able to deal with the additional load.

But I say 7000 is max comfortable trailer on a half-ton.
Doesn't mean you can't pull more: I pulled a 5500 pound 23' Weekender with a 1974 CJ5. There's a real safety factor though when you tow over capacity.