No hate, just my 2 cents. I think that $400-$500 for a mod that will only gain a few HP is a poor investment. In the performance car world, a mod that costs more than $10 per HP ($20 on the high side) is generally considered a poor investment. Headers will cost about $60 per HP, and that is pretty expensive. If you want to make a real difference, put the S/C (or nitrous) on it. You'll get a lot more bang for your buck (though still get beat by the Civic). Headers might make it sound faster but performance and fuel economy gains will not be noticeable to most people. It might 'seem' faster but that is probably mostly psychological (like people who say those tornado intake things make their car faster and get better mileage, even though scientific tests have shown they make no difference at all), or the open air filters on the boy racers that make lots of noise but no real performance difference.
Headers generally don't make much of a difference on modern fuel injected cars (though there are some exceptions, such as a high-performance build with bigger cams). 'Back in the day' a set of headers would add 30 HP or more to a stock small-block Chevy (heads and manifolds were that bad back then!). But today, with exhaust system design much better, better heads, and fuel injection, headers generally don't make that kind of bolt-on difference any more. On a typical LS1, headers will gain less than 20 HP. 8 HP on a 3.4 is probably about right, and that ain't much for $500.
And I definitely wouldn't spend the money on headers if I had a S/C on the motor. Superchargers force feed the air/fuel and purge the exhaust better than headers will do. On 03/04 4.6L Cobra Mustangs, dyno tests have shown that headers make very little difference (like 10-15 HP on a 500 HP motor) until the motor is making over about 600 HP. Unless your supercharged 3.4 is making about 375 HP you shouldn't need headers.
A cat-back exhaust would probably make about as much difference for less money and lot easier to put on.
