I fitted a sports cat to My 220GTi and it was fantastic. I never regretted it at all there was more power and the engine seemed freer although I'd also done some other Modifications as well so can't say for sure how much was down to the sports cat.
Whilst your at it take of the exhaust manifold (header) and clean up the inside with a dremill taking off the lumps of casting and that will also help with power.
