I have had all variations if the milltek. 200cel with res cat back, 200 cell with non res, 100 cell res,100cell non res, decat res , decat non res. Imo 200cel res cat back is not much louder than stock and just didn't do it for me. 100 cell didnt make that much difference in sound although non res did sound meatier. I then went decat non res, it sounded pure
especially when going through tunnels, just too loud on the motorway. In the end i went with decat resonated, perfect imo. Meaty and ripe when you want it to be and nice and quiet when you dont.