This may be a dumb question, but I get conflicting answers. If you were lucky and got it early at GenCon, can you use it for more than casual play?

Generally speaking, no, you can't use it for "official" tournaments until its official street release.

A local store -- or just you and your X-Wing circle -- can have tournaments allowing it, sure. It's not exactly fair, though, is it, for some players (who made it to GenCon or got someone to mule) to have upgrades and ships not possibly available to other players?

Nope. Unless a TO specifically plans for an event with it.