increasing hummers towing capacity

[Don]

I am not having any problem towing at this point, I can pull up a step incline, and maintain 60mph with no problem, and I don't think a couple thousnd more pounds is going to make that much differnce. what I am looking to find out was what is hummer's reasoning for the lower rating? I know in stock engine form I could not be doing what I am doing now without that constant down and up shifting hapening.So is that stock engines lack of power the reason, or does it have to do with other factors that I should correct to make it safer?
Thanks
here is a picture of the current setup, looking to increase boat size from 28' to 35'