The scoob will not suffer in anyway shape or form from a Decat..... the boost won't change as this is controlled by the ECU, there will be no fueling problems, the one thing he will note is that the Turbo spools alot quicker, it wil lcome in lower down in the rev range!
The choice he has to make is whether to go for the 2.5" System or 3" system.... 3" system moves the torque right up in the rev range.
I had the 3" HKS DP with the blitz Nur Spec-r... this made it the loudest car in the world, I sh1t you not this car could be heard 3 miles away, there was always a cup of tea on my parents table ready and waiting when I went to visit them because they could hear it from so far away!!!
There are actually two cats in the system, one in the DP and one in the Centre.... to pass an MOT all he has to do is shove the original Centre section back in and bobs your father, brothers Dads, uncle!
All the best