I've not added headers (yet) but have run the stock 2" pipe expanded to 3" single, with a pair of Flowmaster 2-chamber 40's in series. It was so quiet I took one of the Flowmasters out and it it still quiet.
A single 3" is about the equal of dual 2-1/4" so for most of our builds it is plenty of flow. If you want quiet I'd merge it into a 3" single.
