I have a '98 Navigator with the 5.4 engine.
I tow a 7,000 pound 18' cargo trailer for my work on a weekly basis. I probably tow it an average of about 100 - 150 miles a week - all of the travel stays pretty local, for the most part. The Nav does a good job of this, but if I could do what I'm doing with about a 5,000 pound trailer, I'd enjoy it more.
It's not so much about going slower up the hills... for me, it's more about me not wanting to do the big hills around here and burn up my brakes coming back down!
I am buying a 20' Surveyor travel trailer this year ... and I was very conscious of weights while shopping for this trailer. I wanted something that would be a couple of thousand pounds less (loaded) than what I'm currently towing for work. After all, I want to have fun... and I was kinda hoping a little better fuel mileage and less "work" on the hills here in Colorado would add to that "fun".
My suggestion is that you'll probably appreciate going a little on the light side unless you're planning on just short trips with your trailer... and especially if you're tangling with mountains and such.
For what it's worth... I would NEVER tow the maximum my truck is rated for... the 7,000 pounds I'm towing is plenty. (IIRC, my Nav is rated for 8,000 or so)
Personally, I wouldn't want to tow anything heavier than 5,000 pounds for any distance unless I had a diesel. Just my opinion.