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!
APR's selling point was that you thread the main part down through the faceplate as far as it will go, then glue it to ensure a perfect seal. They argued that everyone's car is different, and so a part made to the same size across the board may result in air leaks for some and not for others.