I installed an X-pipe today and it didn't make much difference to the sound.
Now that the standard resonator is removed and I look into it I realised that it is a straight through x pipe anyway. It simply has some perforations and a resonator so it's not exactly slowing down gases much.
Mines a 2016 Granturismo S auto with the oval tail pipes. I've had the vacuum hoses disconnected from the rear mufflers so my exhaust valves are always open so it was already slightly louder than stock.
Cost about USD$250 and they unbolted the old resonator x pipe and replaced it with just an x pipe which they custom made.
It's mild steel not stainless, so doesn't have that harsher raspy sound. But I must admit I'm a little disappointed as I expected it to be louder and a bit more throaty. It doesn't seem to have made much of a difference at all.
Seat-of-the-pants when driving I don't think I would have even noticed. Putting my foot down it's maybe a little louder than before, but not much. I haven't been on motorway speeds but I used to think in the stock pipes there was a very slight drone at about 100 km per hour and I'm hoping that will be gone.
As you can see in the attached photos the stock resonator with x pipe is a very direct through pipe anyway so I can't see how an x pipe is going to give much of the claimed 8hp performance increase, if any.