Thinking of getting this done and my only experience of Decatting cars was a DPF removal on a E90 330d Beemer.
So standard exhaust with precat removed, does the car drone on motorways. While stopped at traffic lights with windows open can you smell extra fumes.
Also throttle response how much better is it. Had my standard GTI DSG on surrey rolling roads setup and it made 213bhp which seems to be top end of standard figures.