[Lightning]

If you don't want your car to turn the CEL on and "throw a code", I would keep the stock headers (on your car, the O2 sensors look at the catalytic converters that are on the headers) and maybe try "cat bypass pipes" to replace your secondary catalytic converter assemblies. That will up the volume without creating resonance.

Then if that's not loud enough, you could change the muffler out for one of the Dansk units (a Porsche OEM manufacturer, TUV certified, and due to the single chamber/cross-flow construction, no resonance). They sometimes come up on eBay. There is also an internal hack to make the stock muffler louder (search the forums).

If you replace the headers, then you will need to move the O2 sensors to the downstream "secondary" catalytic converters. Some also replace the secondary cats with "sport cats", which ups the sound a little more.

Most of these changes will probably not increase torque. Some may increase HP in the upper range...