I am a newbie here and when talking about towing. However, I am an engineer and can tell you that we all have safety factors built into every thing we design. that includes the stated numbers that are given on tow vehicles.
How much? I can not tell you , but typically it would be 1.5 - 2.5 times the rated number.
Having said that, anytime you are operating on the edge of failure you are too close.
I just bought my first TT. My Yukon is "rated" at 6900 pounds tow limit. I bought a TT that is 4500 pounds DW and 6k gross, although the axles are rated at 3.5k each. I made sure to stay well below any published limits when deciding on a TT.
I picked it up and drove home last Friday and was able to easily maintain 55-65 mph up and down the hills on I-44 around Springfield Missouri. True the trailer was at the base weight, but it pulled easily and I had no problems.