1. maybe 5hp maybe
2. same
3. will net you around 15 combine with the other mods. a dyno tune is the best way to go or
http://www.diablewtune.com/ he is the best in the bizz when it come to the ls4. he will free up alot.
headers will net you good gains but there is only 1 company that makes them and they are almost a grand.
HEADERS at Doug Thorley Headers dougthorleyheaders.com
this is what i did. for about 50 bucks total i cut out the cat, and the mufflers. i left the resonator in place. it gave me great sound and freed up a little power. then i had the o2 sensor tuned out.
stock sound
boostnsrt's Channel - YouTube
just mufflers removed
boostnsrt's Channel - YouTube
mufflers and cat removed
boostnsrt's Channel - YouTube
will @overkill does a cheap tune and right now is the only person out there that tunes the trans as well. i have this tune. it doesnt produce much power, but does improve the drivability, and the trans tune by itself is worth the money you pay.