I read a post on a Mk6 that was put on a rolling road before a remap and was putting out 237bhp before the remap !
I think you're right, particularly in the case of turbo diesels so maybe turbo petrols are the same?. When I had my 118d remapped at Noble motorsport it rolling roaded at 162bhp, should have been around 143bhp. I was told that this was almost always the case with turbo'd engines. I came out with a nice 185bhp.
Anyone been on a rolling road with their GTi and can report their stock figures?