I made the mistake of taking that whole internal bump, bridge or whatever you call it in the exhaust port all the way down. It pretty well stopped the bike dead, but yeah I don't quite know why the piston is so trashed and the cylinder's basically fine.
I have Crazy Wayne telling me just slap this piston in and ride until it dies again (or the next sale whichever comes first). It was probably the overheating that drove the ring into the port. Well, maybe... It will probably sit until I can get next kit. I get tired of blown pistons, and over the past 3 years I've managed to blow out 4 (3 bikes).