We have one fitted too...alongside silicone intake pipe and a Ramair induction kit...
Audibly, there is very little difference with just the turbo muffler delete and intake pipe, but the induction kit has made a nice bit of difference.
The restrictions are clear when you compare the parts side by side...but when will you notice the restrictions realistically day to day driving?? I dont think so...All it will mean is the engine is slightly less efficient...
However, again, saying the above, I have noticed the car pulls harder when above say 80-90mph...and continues to pull very well to around 130-140mph (all on test tracks up in Bruntingthorpe)
BUT...the outlay of the induction kit, muffler delete, intake pipes etc...is pretty hefty for just noise!
I think I would have been happy with drawing the line at just an induction kit...which compared to the old Cossie/RST days, still sound so dull and underwelming!