Again I'd like to make a point a point about the head gasket thicknesses. The silvertop zetecs came with a traditional type ~1mm thick fibre/metal head gasket. Common upgrade is the later Focus multilayer gasket which is closer to 0.5mm in thickness. This will naturally increase the CR and bring the pistons very close to the head squish zones. The ST170 gasket is the best by being 4-layer all steel (like Cossie WRC, but 30£) and 1mm thick.
Also "skimming the pistons" is a bad wording as what you want to do is dish them like in "1.8 esc" 's sig. Preferably not even that close to the edge. There's two reasons for this. First, the distance from combustion chamber to the first ring should not be redused, especially not in turbo engines. Secondly, where the head has two flat areas in the combustion chamber, the piston should have matching flat areas to create the squish effect. At TDC the distance between theses surfaces should be 0.5-1.0mm. This brings it also back to the gasket thickness issue...