would running headers without a cat increase noise as well as overall power? I had an exhaust bypass valve that would bypass the mufflers on my old civic, was so much fun to have that V8 sound if wanted it or just switch back to quiet with flick of a switch....this is not legal in most states though, for "off road use only" lol just had to find a shop to weld them on.

 

I can witness, that ECM tunes for stock hardware exist. I have one of those 'risky' ECM firmwares in my possession. Never tried it on my own LS (neither of both).
This flash increases HP, disables downstream o2 cat monitor, disables evap. Personally I'm not into that kind of funk myself...
It is designed (based) on Euro spec ECMs (so probably won't work at all on US ones due to more complicated evap system etc).
Also, I have heard some rumors, that KESS/KTAG currently supports working with LS ECMs.
To work with MCUs inside ECM directly (no discrete flash chips, two MCUs with embedded flash is used) some rare and expensive hardware is required, BUT I did hear that KESS/KTAG can accomplish as much as raw PCB level dumping, and OBD reflash.
Big stone of knowledge to grind from here, I guess.

 

no civic can ever sound like a V8 🤔