I have the same quandry on my Winnebago f53 which specifies a max of 4 inch drop.
Even though my atv trailer rides a little high, I will stick with the 4 inch max for now. Total towing weight is around 2000 lbs.
For those adventurous/science folks, a max moment can be figured out....4 inch drop x 5000 lbs works out to be 20000 in-lbs. 5000 lbs is the max tow weight.
If you are only towing 2500 lbs, than you could divide 20,000 in-lbs by 2500 lbs, and that would say a 8 inch drop would give the same moment. I have no idea if this is what the Winnebago engineers were thinking, but it is something to consider.