I've done this on my Discovery rally car (std 3.9). It is a lot louder, but not as bad as I expected - fine for a competition car, would be a nightmare as a daily driver.
I removed the centre box for a couple of reasons - I kept bending the silencer, by making contact with the ground, I wanted to help improve flow and hopefully release a bit of power and I wanted it to be bit noiser to stop spectators stepping out in front of me!
I have been pleasantly surprised with the difference - throttle response feels a bit crisper, mid range it feels like it has more pull - but it could all be an illusion, I haven't had it on a rolling road and can't really quantify.
It does pop and bang a bit more now too.