This has always baffled me and I've asked the question before on the mkiv forums when I had a Golf GTTDi and rang numerous tuning specialists and had different answers from all of them.
Obviously a standard remap is set to run with standard existing parts, I know the only thing that doesn't make a difference is changing the induction which will increase the performance by a small amount regardless.
So the 1st argument is that you would lose performance as the maps set to run with the cat on the car. You would then need a custom remap to get the extra performance from the decat.
The 2nd argument is that how are you able to lose performance when your letting the gases escape quicker and letting the exhaust breath more freely.
To this day I still don't know the answer and would love to know the official answer. Anyone have a standard remap and a de-cat and did you gain or lose perfomance?