With the stock seat the platform angle leans you backwards slightly. This will cause your back round over issue exactly as you are describing. On a long ride without a windshield this will also cause pain in your hips because you will have to use the muscles in your hips to counteract the wind.
The long haul seat has a different platform angle, and will cause you to sit more straight up. This corrects the seat angle issue, and alleviates the need to hold yourself up by the hips. Unfortunately the long haul seat is not as wide as the stock seat, and in my case it caused numbness in the center area of my butt after more than 15 minutes of riding.
My solution was to have a custom seat made on the stock seat pan. I combined the best of both the stock seat, and the long haul seat, and ended up with a new seat that is superior to either of them. So don't let the seating position stop you from buying a Thunderbird if that is your only complaint. It is easily corrected.
Thanks man, I will definitely keep this in mind. Probably not getting the Bird til Spring, but if the ergos don't get better (not to say they were terrible in the first place), then I'll look at other seating options.