You can leave the stock exhaust (or close to it) and still get good power. Your noise level is going to mostly be disctated by the catback you run.
I have an STi TMIC, a VF39, a Cobb Catted downpipe, a stock midpipe and an STi axelback. I'm making 280whp/312wtq (conservative tune) and my car sounds stock when crusing on the highway. It is very family friendly and my wife has never complained.
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05 Forester XTi Premium M/T (PRE tuned VF39)( his)
05 Forester X A/T ( hers)
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