A friend has asked me to polish the top of his new Dt 360 piston and the combustion chamber of the MX head he is also going to install with it. He wants a "mirror finish" on both. My question being, will this have any difference/ benefit. to performance. It is a stock Yamaha replacement DT 360 piston and a stock 360MX head. I have seen photos of many engine builds local and Pro and have not seen this done on any of them. Opinions please.

Not sure if it will help performance noticeably....  carbon won't stick as easy though.  I used to do this all the time.

Unless exhaust port is raised, I would not use an MX head on the DT.  Compression will be too high. 

Yup when 16 on my yam fizzie, I used to lift the head and barrel every other week just about, in order to polish the combustion chamber, piston top and clean the carbon out of the exhaust port.

Cannot say if it made a difference, but at that time, I thought the polished surfaces would reflect the heat back into the fuel mixture and make it burn a lot faster.  I thought that was a good thing.....