To get the most from an exhaust you need to address the full system.
Header (or headers), high flow cat or test pipe, larger dia. pipe, high flow muffler. Of course to really maximize exhaust mods you must address the intake side too.
Even with all of this work, you'll get maybe 10whp and have spent a lot of cash.

A note on exhaust piping dia. Make sure it is a suitable size for your vehicle. I won't get into any crazy technical talk about it. In my personal exp. 2.25" works perfect for 4cyl vehicles, 2.5" for 6cyl vehicles and 2.5-3" for v8 vehicles.

 

K&N will refund the cost of the FIPK if you dont see 9HP increase (on the 3.4)
i havent done it, but its there...

 

Well, we got it done. Sounds very beefy from 2750rpm up....especially nice at around 4K. At is does have more pickup, noticeable enough. Pulls harder from 50mph, when you floor board it to 2nd gear. That is with a stock filter. Getting my trueflow soon! SOUNDS MUCH BETTER>

 

youll get a little better response, sounds, maybe some hp gain, but i would say no more then 10hp, and no less then 4hp...then again, its not gonna be consistant thru the rpm range. you will prolly see the highest numbers around like 4.8k to like 5.2k

 

i would think you'll get at least a little better throttle response