I posted this above but it bears repeating. The bigger restriction in the stock downpipe is the flex, not the CAT. The inside of the flex is about the diameter of a quarter. In addition, the bend coming off the turbo is far from optimal.
Having repeated that, I'll also repeat that you'll not see a significant benefit from an improved exhaust until your mod levels are beyond a tune and injectors. When my car was in the 180 HP range changing to a 2.5" exhaust got me 8 HP. On a more mildly tuned car the increase would be far less.
2002 Jetta wagon, 345K, RC3+; 1993 Mercedes-Benz 300D 2.5, 197K; 1997 Passat, 283K; '99.5 Golf, 250K; 2011 335d, 48K. Principal, http://www.idparts.com
Kid's cars: 2002 Golf TDI, 2002 Jetta TDI