Just like to chime in with my experiences regarding the Insight and cargo capacity / aero trailers.
Although it's only rated from the factory for something like 450lbs of cargo + passengers (two large passengers can exceed that!), I've seriously exceeded that before, and had no issues with ground clearance, handling or power. Braking is another matter, but taking it slow really helps and hybrid braking makes a noticeable improvement.
I recently used my Insight to move from Florida to Vermont, and drove along the spine of the Appalachians with the following:
Myself, my wife, my border collie and my sheltie
~500lbs of stuff in the car
trailer loaded with 1000+ lbs
Total estimated in excess of 3500lbs, with the car coming in at ~1850lbs
We drove to the top of Clingman's Dome (6,650ft, 4,500ft relief), frequently WoT in 2nd gear @ 4000+ RPM, and had no problems with cooling even with an upper grille block. Going down the other side I experienced no brake fade, despite the grade being very steep at times. When on the open highway I was comfortable keeping up with traffic when necessary, and still averaged 57mpg with a target speed of 60mph over the 1,500 mile trip. Braking distances were a big long for comfort, so I kept my distance.
