The seam is noticeable on mine, but only from looking at it, not from any performance aspect. It's a pinned rim instead of a welded one, which is why there is the more visible seam. If you look at most factory wheels, they just cover the seam up with a sticker so you don't notice. Plus, the seam gets worn down by the brake pads over time, and becomes less noticeable.
I would imagine the fact that you sanded your rim down with steel wool would preclude you from any sort of warranty type service, unless he was feeling mighty generous.