Im going to have to say that is FOD (foreign object damage). The marks are to square edged to be detonation. Detonation is pretty much always domed in shape. Also the fact that you had stuff in the exhaust points to something being in the cylinders. Had you had intercooler piping welded up and some slag could of broken off and gotten into the cylinders? Or are you running an open intake on the turbo that you could of sucked something in?? Just my experience.