You and I have experienced very similar situations. When I moved from Killeen, TX. to the Chicago area, I hauled a 6x12 dual axle trailer with the bumper hitch. I was getting reimbursed by the pound, so I wieghed everything empty and full.
Truck and trailer empty - 6000 lbs.
Loaded truck and loaded trailer - 10,700lbs.
I never got the chance to weight the trailer seperately, but if my truck weighs about 4,700lbs (4.2 SC 4x2), and I did not overload the truck payload cap. (1700lbs), that puts the loaded trailer in the area of 4,300 lbs.
The bumper showed no signs of stress, and the truck moved that weight pretty well. I had some trouble with the CC, but not too bad. I think the catbacks really helped; if it became a regular thing to tow that much, I will go with a chip. As far as ride goes...it would be much better with a real hitch. I could probably go as high as 6,000 with a hitch, but any higher, and I would need a V-8. I don't know if I could stomach the 5mpg drop though.