I went down the Milltek route too after considering others such as APR/STASIS
Initially I was running the stock cat and stock map and as you can imagine I didn't notice any performance gains however the exhaust note sounded deeper but not intrusive, internally you could not hear much difference. I then swapped out the std cat for a de-cat pipe running the stock map. The difference in performance was noticeable it wasn't a big difference but there was a definite increase felt in the torque department. The turbo seemed to be spooling slightly quicker and the throttle response was a bit sharper. The Milltek sounded louder, more meaty/raspy but still the cabin noise was not too intrusive but more noticeable inside.