First I would agree that a PC would be a bit too formal, if you chose not to wear the kilt you wouldn't attend as a guest in a tux.

Before my kilting days I attended a wedding, and a guest did come kilted. Sat at our table all night, and to be honest it seems like no one even noticed (or cared) most of the time. Certainly no one said anything. He was the only kiltie in attendance.

And as for upstaging the bride and groom, I don't really think simply wearing a kilt would upstage them. It's usually the drunk guy making a scene, or a family dispute, or (most weddings I've attended) the one trying to steal the show was actually the DJ. Unless you are recreating the famous Braveheart "mooning" scene, I doubt a kilt would upstage the bride. Most eyes are on her from the beginning anyway.