What are your goals? If you are hell bent on NA most power, go read the internet until your eyes bleed on all the 3800 forums you can find. You likely will come up with tons of dollars spent for gains that could be had cheaper/easier on different options like turbo/SC.
I'm not going to sit here and tell you that the car sounds bad based on you straight piping it. My Regal with GTP catback (no res, high flow cat) sounds amazingly fine even on the throttle. It's an oddball. Most 3800's with any exhaust mods (and stock) sound like crap. They have rasp and drone. Your's might be an oddball too.
Many will tell you to turbo or top swap etc. Personally, I've learned a ton from this platform....and broken more than I like to admit. That's expensive and at the end of the day I gave up on the platform and sold off the performance stuff. There are many other cars that handle/make power better than the 3800/4T65 combo. That's not saying it doesn't do well for the mass produced setup that it is.