Would a DC 4-1 header combined with a US OEM ITR exhaust system be a decent set up on my 00 GSR? I'm not looking to make noise, just improve performance, and maintain a "stock" look.

 

You won't gain anything on a stock GSR with a header and exhaust, except maybe 1 or 2mpg...

 

I may be installing CTR cams when I service my timing belt. Will the header/exhaust help then?

 

drop the dc header idea and use the 98+ jdm itr header, much better piece. other than that, who knows, few hp, improved sound. coupled with R cams and a single runner manifold the performance increase is substantial with a tune 20hp would not surprise me at all. you will also need a tune and should run a p73 map for a get me by till you tune it. also, don't cheap out and not change the valvesprings when you do the cams. it's all of $50 or so for itr intake springs, you then move your gsr intakes to the exhaust and wind up with a correctly matched valvetrain.

 

I was already thinking about the skunk2 manifold as well, I've heard its a great improvement over the stock GSR manifold. But I've heard the JDM ITR header wont just bolt on, it requires customization. And some say the DC flows just a well as the ITR.

 

I ran a DC Sports 4-1 on my GSR with a Comptech exhaust. With GSR cams and the GSR manifold I was at 174whp I think. It's been a while.
Then I had the head done with CTR/ITR cams still with the 4-1 and GSR manifold it made 182. At this point I maxed out the ability of the manifold.
To answer your question, yes, and with tuning you will make even more power.

 

edelbrock performer x is the best 8500rpm and under manifold out right now. it's better than the skunk in every way except easy of install. also, the jdm itr header is a total bolt on. it's the 2.5" collector that could be the mod area BUT you want a 2.5" collector since you'll have an itr exhaust and are going for power. DC also uses some knock off ceramic, jet hot it is not. it's only the stock gsr cat the jdm 4-1 won't bolt up to. a jdm cat or 2.5" magnaflow with the correct flanges is a perfect fit