Have been following this thread, and thought I'd add my completely unresearched opinion for what it's (probably not) worth.
It's been mentioned that the exhaust valves are closed in a certain rev band, isn't this done to generate some kind of back pressure until the turbo is spooling enough that it then needs to get rid of the gases quicker, in whihc time they are opened?
From memory and younger days a lot of people started changing exhausts to bigger ones with less boxes, but it was found the turbo pressure had to increased and engines started to go bang quickly (RS Turbos).
So the question really comes down to how does this mod actually effect the engine and turbo rather than just increasing the noise? because if it's having to up the turbo pressure to account for the open exhaust, and as mentioned it's logged that the valves are disabled, that's your warranty out of the window, and a very expensive engine replacement in the worst case, or a turbo replacement as the seals are blown.
Like I said, maybe fine, but also maybe worth some kind of test to see how the turbo pressure changes. Also depends on the length of ownership I suppose