That's right, they don't make any or a big difference, by themselfs.

 

I ment overall, I mean I have a AEM CAI, the head's, a high flow cat, and the mbrp exhaust. And after putting the stock heads back on I didn't really notice a difference at all except in sound. ???

 

the headers would only help out on the top end, so actually by puttin on the 4-1's you would lose some low end torque due to the wider diameter of the piping causing less back pressure... compared to stock
but with them on you should be able to feel extra pull over 5500 rpm .....
i have stock exhaust manifold and the mbrp catback and i leave for work @ 4 & 5 AM, and i know wake up the neighbors in my apt building cuz my exhaust is so loud (mostly deep tone), i feel bad so i wouldnt want to put on those headers cuz i know that would make it crazy loud ...(i dont think theres much you can do to aftermarket headers to quiet the noise, besides turbo) i dont even wanna put in my VHR kit yet cuz ill have to rev so high to get it moving ......