For a 54 X 54.5 config 0.65mm is the mechanical limit , even then there will be a 'shadow' on the piston where it has almost touched the head on over run.
On our 98 PULP running 15:1 is no problem provided you sort the squish. There is more to it than the just the gap the width play a big part as well. Lately torriod heads are being touted as the new black as a 'dual angle' squish bands