I have Xpel on the M3 ever since march of this year. I detail it with Chemical Guys Clear Seal detailer everytime I wash the car. The line is visible if you are looking for it otherwise hardly visible. Since Xpel is self healing and has clear coat built into it... It has been scratch free and as clear as day one
My other two cars are a white VW CC and a white FJ cruiser with either venture shield or 3m on them... Scratches in those show, it's not as clear, and it has clouded/ yellowed a little. Might be due to the fact that I was using Protect All or Plexus on it which was originally recommended by the installer. Turns out the petroleum ingredients in those products will start to stain the clear bra after a while.
I was able to correct those using clay and chemical guys V36 followed by M-Seal and now regularly detail it with chemical guys clear seal detailer. The correction using an orbital polisher took care of the haze, scratches/ swirls, and took off the yellowing... It's much more clear but still not as good as the Xpel