The J2087 consists of many tests*. Flat accel (0-60, 0-30, 40-60, etc), incline starts, parking brake, long climb (Davis Dam Grade), braking, stability, hitch, etc. The final TWR is the lowest result of any test.The limit is a limit for a reason, and now with this new SAE standard, it should be especially so.
*website doesn't mention all tests
As a made up example, say the Davis Dam climb resulted in a 4700 lb TWR, but every other test came up with 5500 TWR. Personally, I'd be fine running 5500 lbs. The Davis Dam climb is a 7% grade that goes up 3000 ft while starting at a minimum of 100 degrees ambient temperature. Also, the A/C is on high and the trailer being towed is enclosed with a minimum frontal area dependent on the weight class the TWR falls into. These are extreme conditions that I can avoid by route/trailer selection.
One factoid regarding J2087 is the trailer frontal area goes up significantly if you go over 5000 lbs TWR. I'm sure it's no accident it lines up with the limits of Class III hitches, also 5k lbs. A 5000 lb rating requires a Class III hitch and ~50 ft^2 trailer. 5001-7000 lb rating requires a Class IV hitch and ~70 ft^@ trailer. I'll have to look up the actual figures Monday when I can look at the paper again.