Congratulations.... Love the other cars too.
The Accufab/Heffner etc X-pipes are much louder than stock, especially from inside the cabin.
I keep going back and forth with my Accufab (also it is so loud that you are not allowed on any of the tracks here in the UK) and the stock set up.
When I have the X-pipe on I think it's too loud (I tend to drive the car a lot, and on a long journey it gets tiring), then when I put the stock on I miss it.... oh well.
Where in London are you located? There is a bunch of us here... ThatPhillBrettGuy is also in London (Wapping) and Neilda is down closer to where I am (he's in Godalming, i am in Weybridge).
Anytime you want to get together.... it's another excuse to drive the GT....