Milling of a warped head, causes the compression on the end cylinders to get higher since the volume in the combustion chamber is reduced with each milling. Normally the head warps downward on the ends. The volume of the end combustion chambers becomes smaller, the more the head is cut. I have seen compression on some end cylinders as high as 230 psi while the center cylinders were still around 170psi on a severely, over-milled head. Correct ignition timing would almost be impossible on an engine like this. End cylinders might have ignition pre-detonation (pinging) while the inner cylinders were fine. A radical, dangerous fix might be to measure combustion chamber volume and remove some head material from the combustion chamber in the head until all combustion chambers are equal. However since the valves are moving closer to the pistons after each milling, you could have problems with valves and pistons crashing into each other. In addition reducing the thickness of the head slackens the timing chain by twice the amount. Best to stay in spec. and you'll sleep better at nights!