I think it's great that you're looking for ways to help support the site and giving something to members as well, but as someone who runs two websites (both using vBulletin with forums) I know that all the features you mentioned in the OP are just a few clicks of some options. Considering the amount of bandwidth the site probably consumes I can give an pretty accurate guess of what this site costs you to run and, well, I'll just say that you'll be on the plus side.
That said, I think some of the ideas of giving members other things like T-shirts wouldn't be a bad idea. Have you also thought about maybe printing a newsletter that is CP related that premium users would get? If you don't want to do the work of a newsletter yourself (or don't have time) how about the newsletter of another CP related site or organization?
I'm still new here really, but maybe even discussing something with the NASC founder about finding a way to bundle membership since it doesn't seem like NASC has their own forum community.