This is my Duster, I run Doug's headers into the Summit 2.5" system with Dynomax UltraFlow mufflers.
Not so sure I'd go 3", especially since you say "low noise" and "discreet". I can tell you there's nothing "low noise" about my car's set up, it flat out roars. Even right around idle it still has the tendency to set of car alarms as I drive through parking lots. And with the windows down on a long trip I wouldn't want it any louder. It's not terrible inside the car with the windows up, but I also installed FatMat insulation on the entire floor of the car. And you'll want tail pipes too, if you dump it under the car it will be much louder inside.
As for power, you won't lose anything you'll notice either way.
