I did a decat on the corsa while I had the 3" system on it, blew the cat clean out the back and it was stupidly loud even driving slow. under 2.5k could be heard 4 miles away behind closed doors. popped and banged like crazy on run down.
2.5" down pipe with 200cpi cat and decent 2.5" system. mines back to this now until I save for a 3" system to be made.
this is with 2-3 layers of sound proofing inside the car, more on the doors and floor
get loads of complaints about it being to loud.