well really compression = pistons as there is some of the compression difference... also the head gasket is a different thickness
the crank and rods i believe are identical.. where it differs is in the pistons... also it's missing the oil squirters that the turbo block has and the sump is designed different for the oil return for the turbos and the non turbo block doesn't have the feeds for the turbo's
that's about all i can think of off the top of my head... also the head's i would assume are different... but i am in no way a mk4 expert so do a bit of research
good luck