This forum was actually started at forumotion (basically a free forum hosting service) by another passionate GT fan and I saw an opportunity to partner up with him to launch something significantly better. We were initially going to launch with either phpBB or FluxBB but decided that we were in it for the long-haul and wanted to start with a more forward-thinking forum platform and Xenforo came out on top.
The migration was done using a scraper python script that I found at phpBB to convert forumotion into phpBB3. Then used xenforo's import functionality. Once we converted everyone over to Xenforo, we asked all users to get their password reset since the two-level migration didn't maintain passwords. The problem was that apparently forumotion recognizes scraping attempts and converts the email addresses into an image to prevent scraping the data. We were able to reset the email address simply by copy-pasting to a google spreadsheet and generating "update" SQL queries that we ran manually on the xenforo database to update all user email addresses.
Anyway, I'm very glad that we decided to go with xenforo. It's been extremely easy to use and customize.
Check us at at http://gtswarm.com/