Think you've got the wrong end of the stick there Robin. He wants to know if the OEM system fits the standard GTI. If he goes to Milltek he'll just get a GTI one (although after the CAT the only difference between the two milltek systems is the tailpipe arrangement).
As for fitting the OEM ED30 item to a GTI, I'd imagine not a straight swap, but anything can be made to fit with the right tools and determination.