Hi mate,
The basic diesel emissions test involves testing the amount of smoke not co, often by shining a light through it and measuring what makes it through.
The de cat shouldnt put any lights on as its not a proper closed loop system and has nothing to monitor the other side. eg petrol appliations the first sensor makes sure the lambda is good and the second, after the cat checks the cat has done its job. If you see what I mean
Im also thinking of the decat for my gt. I have come across 2 off the shelf options, milltek or THS. The milltek is left 63.5mm at the end wheras the THS includes adaptors to take it down to the 2 available sizes of centre section if need be. The THS is significantly cheaper and quality looks just as good but nowhere has any
TBH I doubt there is much changed on the map for this. Maybe a good question for your mapper, I also wouldn't mind knowing what they change.