A while back, I had '94 Bimmer, the windshield was pretty scratched when I got it. I ordered a windshield polishing kit from
www.eastwood.com. They have many tools and kits for auto restoraton and modification. Anyways, seems the bimmer glass was fairly soft, the polishing kit left swirl marks in the window, similiar to paint. I am sure that there is some technique, very similiar to buffing a car with a power buffer. Probably best left to a windshield shop.
The official BMW fix for window haze is to use Bon-Ami brand cleansing powder. It is very fine, does not contain soap, and was mentioned in a factory tech bulletin.
The Bon-Ami worked well for cleaning the window, but I don't think it will remove the scratches you are talking about.