Its possible that the engine was blowing plugs because it was rich. If you were barely pushing 200f it was running rich and if it went flat on the track its possible that the engine was breaking in completely, It would have had to have gotton to 350f before you would have lost all power due to being lean. Adding a extra .1mm shim should help. As far as the piston being black, thats more of a fuel issue and not necessarily a temp issue, I have seen engines come of at 300f+ and pull the head off and there wasn't any black on the piston, and I have seen engine run perfect tune for a gallon and the piston is completely black.