I've never seen anyone in person throw significantly farther than I throw. (Other than at Worlds last year, that is.)
But I live in a disc golf void. So it's not surprising that my distance rivals those that I get to play with.
So, if I am going on 50, have decent form, am in decent shape, and can throw 350' for real... There's no reason for me not to believe that younger, better conditioned athletes with better form can't throw over 400' regularly. Heck, eliminate age from my equation, too.
I'd love to see some long throws. I just have to get out and play with more people and I'm sure that I would.