[Henry Schmidt]

The proper way to machine the heads is with a fixture in a lathe.
Porsche calls for .25mm maximum to be removed from a head but you can get away with a little more. You just have to compensate for the material with thicker base gaskets.
The thicker base gaskets mean more deck height which offers it's own set of issues.
You might be better off starting with a new set of heads. 2.7 heads are relatively easy to find and generally cheap.

At the very least it sounds like you need one replacement head.

Call, I may have what you need.