Definitely add the sugar. Don't precook the orzo, it tastes so much better cooked in the soup and doesn't take that long. You have to make the soup in a huge stock pot, though, because the orzo will like double the total size.

I didn't try any butter with the tomatoes while they were roasting, but I tried throwing the basil in there with it and learned that roasted basil does not taste or smell good. I had to strain it out. A little olive oil might be good on them, but butter might be overkill.

If you don't want to blend some of the ingrients with an immersion blender, you can add a little cream for the same thickening effect.

The basil and white wine yeild a really fresh and sweet soup, I think oregano would not be good in it.