Whereabouts are you located? If you have a 'euro' engine, ie the option 822 300hp engine, then its already got quite a high compression, and I wouldnt really take it any higher.
I have one of the 5.6 option 822 engines in my 500SEL running on LPG, and its great!

If you really want to take the compression as high as it will go, I can look up the specs later when I get home and let you know how much you can skim off.

Valve timing is quite easy to adjust and from memory offers upto 12' of adjustment in either direction, by installation of offset keys on the cam sprockets.