What was your total advance at idle and head temp with retarded timing vs total advance at and head temp with more advanced timing? I was taking you post like you were referring to EGTs, with a lot of timing pulled the egts can be 1300-1400, but when you crank some timing in they ‘cool’ off a bit to 900-1200, even though nothing is really cooling off. With the retarded timing the a/f mixture is burning going out the pipe resulting in a false, high reading. All the while the plugs still look brand new. Advance the timing a tad and the heat stays in the chamber where it belongs, the egts drop but the plugs are starting to show some heat. Egts are good for diagnostic purposes but I wouldn’t really rely on them to balance cylinders and put faith in them for how much to lean it. Just a side note, EGTs vary if they come in from the top or the bottom and vary depending on how far they are in the pipe and how far away from the cylinder they are.