Smokem and CAHAMMER are right, 20* plus is pushing things on a stock head gasket. I am running 16.5* no poblems and it works out good, but 15* is probally a good start unless you plan on studing it!
i am running 6mm of plunger lift on my 912 pump. i think i may have to back that off a bit. i thought it had a bit more timming clatter than i normaly have. it figures out to be in the 21.5 range. it is a real push to have it that high on a 260k head gasket with stock bolts and 60 pounds of boost.