If I were you, I would get a Tune first!!! It will be the best and biggest bang for your bucks!!!
Headers add very little in HP and a little more with a Tune.
JDM is the place to go to for a Tune.
@JimIII@JDM
Short or long headers will throw a code and you won't be able to pass a emission test.
The rear O2s will be in a "not ready" state when plug into the emission tester.
You have the option of adding rear O2 extension with different orifice washers,
so the rear O2s will heat up in the correct time the PCM's emission tests is set for.
With a Tune,
You have the option of using the rear spacers and keeping all the safety features and pass the emission test.
Or just have the tuner turn them off so the check engine light won't come on.
But then you'll have no safety if a cat goes bad and you won't pass emissions.
There are differences with cats as well.
Stock cats are the best.
Kooks Green Cats are the best replacement.
Hi-Flow cats are the worst and won't last long.