I know the ones for the biposto S1K were aluminum with "race only" stamped around the end ring vs stainless with "e1" and a bunch of numbers for the homolgated ones. Both have removable db killers, but the stainless ones also have packing inside and I believe cats. I know holding homolgated can up and looking down the end you can't see all the way through whereas I believe in the race version you can.
That being said, no idea what they made for the GT