I have managed to warp my head, and it needs about 20 thou taking off to make it flat, is this going to have a big effect on the compression ratio?
will i need to get it re-mapped?

 

Take the head to someone like Ferriday Engineering and they can machine combustion chambers to keep same comp ratio.

Or use a thicker head gasket to keep comp ratio same

Don't forget to measure piston to valve clearence after skimming if you stick with std head gasket.

 

Go for a fuelling check as this will also show up any det caused by raised CR, 20 thou(0.5mm) is not a lot in terms of the swept volume of the piston into squish area of head so chances are unless right on limit of performance boost all ok.
tabetha