Nice s13! My buddy just bought a JDM s13 coupe. I'll send you some pics when I get some.
My header is fine and I have a full 2.5" exhaust, which is big enough for the power that I make. I really don't give a fuck about YOUR opinion on MY exhaust. What makes you think my exhaust is......"dinky" anyway?
For not only the pure fact, people come to me to make their cars go fast, but from the vast knowledge of learning exhaust flow and velocity..
That "DC" header, it's primaries are no bigger then a stock usdm itr header, which is roughly too small for any type of power or forced induction setup..
Secondly, that "DC" header, has a 2.25'' collector, which is "bottlenecking" your exhaust flow.. hence losing power.
Tell you this much tho bro, exhaust shops, and header shops measure the outside diameter of piping, so that collector, is really smaller, and that exhaust your using, is really 2.3/8ths instead of 2.5''

Go measure, you'll learn something.
Lastly, i'd put money on you getting a better header, and a "true" 2.5''+ exhaust you gain almost another 20+whp if not MORE on your setup.. which is pretty stout.. but if u dont see the gain of spending what, 100 bucks for a true exhaust system and another 300 for a "decent" header.. 400 bucks for 20+whp? to the ground? Seems kindof stupid if u dont do it..
What do i know tho, right? I'm just some dude, who people pay to make them go fast..*shrugs*
I'm giving you corrective crisitsm, to help you go faster.. i've been doing this since i was 14, and i'm 25.. i've seen what makes power, i'm seen those "Freak" cars.. hell i had one.. Point is.. when you gain 17whp on just bolting up a header on a all motor app like i did.. hehehheh you tend to start wanting to know why you gained that much power..
I ran a header like yours a JSPEC itr replica header, with a "2.5'' collector.. to a 2.5'' exhaust.. it was cool, made 207whp stock block mind you.
Then i got a 2.0 hytech.. REAL hytech.. with a tsudo true 3.0'' exhaust.. and picked up over 12 whp w/o tuning..thats a all motor setup, on a STOCK block.. which is now getting stroked.. whats funny is, i'll be making as much as you are now, all motor and off the juice..
and i have a 75 and a 100 shot, both direct ports..
Do you know how nitrous works? or should i school u on that too?