Back pressure.

Most exhausts from the factory work on a small amount fo back pressure to control some emissions (helps keep some exhaust in cylinder after combustion to prevent new fuel/air mix from getting pushed past exhaust valve). Works great in N/A engines.

On turbo cars you want a very little amount of back pressure to force all that exhaust into that wonderful snail to put some nice fresh air in your combustion chamber. Better the flow the more efficient the turbo and engine generally resulting in better fuel economy. Happened to the new ford F-150 EcoBoost engines, too much restriction was causing Check Engine lights for too much back pressure on cylinders. Ford put on betterm higher flowing exhausts and the problem was fixed and fuel milages returned to their usual claim of 9.8L/100KM, 29MPG in Queen's English, in a full sized truck.

I would check some companies like Flowmaster as they have a low amount of sound until you really hit the higher RPM's. I run one of my XR and it's great not too loud until you really get into it. It's a full 3" inlet outlet exhaust and I have only a downpipe, muffler(silencer), and full 3" mandrel bent pipe out the back nice simple and fairly quiet. No 70MPH hum.

Hope this helps some, I'm sure there are some Technicians on here that may have some products that may help.

Happy Motoring!
TMitch