If your head was "banana warped" you better know how much they took off and it better only have been a few thousandths.
When a head is warped end to end and you only plain the bottom the top remains warped and the cam now must pass through the warpage. It don't like this. The proper method of dealing with more than a few thousandths is to heat the head and press it relatively straight - to within a few thousandths. then mill it.
You should place a staright edge on the top of the head. Hold it down on one end and measure the other end, This is double the amount that was taken off. It better be less than .005in.
Bosch Master, ASE Master, L1
33 years MB technician