On May 1st, my wife and I packed our 50lbs Border Collie, 15lb Sheltie, and everything we owned into the car and trailer. We pulled out the carpet and secret compartment box, and stuffed the spare tire area and other unused space completely full, with items like blenders, books, and clothing to fill in the gaps. The cargo area was packed to the roof, aside from approximately 2' x 3' for our Border Collie, and the Sheltie sat in my wife's lap. She had items around her feet, and we packed full the spaces beneath and behind our seats. We packed the trailer full too, with not enough space left to fit a book. I left my tools in an easy-to-access spot in the hatch, in case we had any major disasters.
I can only guess what the total weight was, but I would be surprised if the trailer had a pound less than half a ton, and there was at least 500lbs of stuff in the car too. With my wife and I, and a full tank of gas, we were probably at least 3500lbs total.
Our route:
Even under full assist, the car accelerated at a snail's pace. However, braking distance was still reasonable, and at no point did I experience any instability. I loaded the trailer so that the tongue weight was only around 50lbs, and handling was excellent.
We stopped the first night to camp at the edge of Great Smokey Mountains National Park.
Getting up to the top of Clingman's Dome required 2nd gear WoT almost the entire way. I kept an eye on coolant temperature, but it never exceeded 197F. The brakes held up great during the rapid 4000ft+ descent, and despite some serious potholes which scared the living hell out of me, the 2x2 and corrugated plastic trailer survived without any damage.
Even cruising at ~65mph when the speed limit allowed it, and frequently in 3rd gear at WoT to get up one mountain or another (I-81), I still averaged around 57mpg for the full trip pulling double the car's normal weight.
I consider the trailer experiment a success, but would consider upgrading the brakes if I frequently pulled close to a ton of cargo.