I've never been to one of these so i'm not sure what to expect.
I am going to the roadshow on Monday and Iwould think it was a smart-casual affair as it isn't an interview or anything. However in my opinion, wearing a suit is not over the top. I think that's what i'm going to wear. I can't see anything wrong in turning up smart. Better wearing one than to attend and find yourself the only one not wearing one!
Also, lets say you asked a very good question or made a good impression on those who are there who may be important. You would also want them to remember you by your well presented appearance.