Strange that many say it 'made a huge difference' but can produce no dyno graphs showing gains.
I've seen S3 de-cats dyno'd, and tried std cats, 100 cell and 200 cell cats personally (you can see through both of the latter clearly) and found no real gains at all - dyno proven (that is a few lb-ft...but within the tolerance of the dyno, not concrete proof of a gain).
Also, many who de-cat have problems with engine check lights coming on.
So, an engine check light, a lot of hassle to get it through the MOT, getting nabbed at a roadside check if you get stopped and no power gains...
Worth it?
Not in my book.
Never mind...it's another meaningless modification for people to add to their signature to make them feel better by an amount directly proportionally to the amount their wallet has been lightened buying the de-cat. Good show.