Back in 1977 when my 250C kept eating pistons, fitting 2 base gaskets was the only way to cure it. No loss of performance that I could remember at all.

Fitting 2 head gaskets would make it difficult to seal and probably lose some compression, whereas no ill effect going the other way.

As for telling whether a head is modified or not, why not post a picture? May be the easiest way for folks to check and tell you.

Originally posted by ting if for thoughts sake, the piston was hitting the cylinder head, would an extra base gasket or head gasket or both give you that little bit extra space , or would performance be drastically affected by this and is their any way to tell if a cylinder head has been modifyed

easy way is to look for a tiny lip or step where the squish starts.
std has one. modified may still have one but usually doesnt.