Exhaust performance

[sheerwater]

Sorry to butt in on the performance boys but maybe somebody can give me advice on my exhaust. 1997 940 estate b230ft engine.
I had an exhaust fitted under warranty at my dealer. They stated that it was the third attempt to get an exhaust that would fit but Volvo had assured them that it was the correct one. You dont take much notice of exhausts really but what was noticeable was that the tailpipe was much smaller bore ????? Funny I thought as Volvo kits come as an out of the box as a bolt and fix kit made for the job. So why the small tail pipe.
Measuring the exhaust starting at the turbo which is the original 3"
through the cat the pipe then starts to get smaller down to 6cm then it goes through the first box. It then flattens and ends up 6.5 cm wide and 4cm deep before reverting to 5.5cm . The tail pipe ends as a 5cm. Now looking at this from a laymans point of view having a large turbo outlet pipe of say 7cm gives a cross section of 36 sq cm. If the pipe narrows to 5.5 cm the cross section is 24 sq cm and then to a tail pipe of 20sq cm. I would consider that the flattened section is probably less than this. Now I know exhausts are not perfect having to change through joints,bends,boxes etc but if the engine needs a 7cm tube out of the turbo why narrow it down to 5cm at the end of the system? Has anyone any comments to help on this one from experience.Much appreciated. Nigel ps all dimensions approx