Stock exhaust is free flowing enough. Only thing that will net you more power is to gut the cat, which being in California is out of your possibility.
Last option is a high flow magnaflow cat, but the gains will be small, maybe 5HP at best. Don't waste the money unless you are failing emissions and you KNOW its the cat. Spend the money instead getting a tune up and all fluids and filters changed.
If you want a throaty noise, you can always punch a hole in the exhaust before the muffler...
