You don't know until you try it. You will find lots of room for another muffler= put your 1st muffler right up to the flex & another one behind that. Add a resonator next to the fuel tank. Some guys have 3-4 mufflers w/ resonators. You having a '09, Have you heard or drove a stock '05-'07 ST? Mine sounds very similair to stock.
Introduce Mr. Torsen to the mix
Bigger brakes then you- D23 crew- D-Tech- Sedan Revolution= #131