Have you heard good things about that company or had previous experience with their products? There are many companies out there that make headers by copying popular designs, this is fine but what you need to look out for is if they are using real SS or some other metal. Mandrel bent is also what you want to look for since it will have smoother bends as opposed to a press. Finally the quality of the welds, that is the biggest concern with getting aftermarket headers. The one you linked seems to be good, I'd just be wary if you've never dealed with that company before tho.
As for SS or ceramic both will eventually deteriorate overtime. The ceramic header is a mid grade steel with usually a ceramic coating inside and outside the pipes. The ceramic coat will eventually peel off and expose the mid grade steel which won't hold up that great being fully exposed, but this is a very very long way down the road. The SS header is more durable steel and can be repolished to get it back to its original appearance, looks much better than the ceramic header imo. Pros of the SS header are durability, looks, hotter temp which will help with exhaust gas escape velocities (hotter= faster moving gases which is better); the cons are that the engine bay will be warmer but I have the SS header myself and I've never had any issues with heat since we have so much airflow and extra space in our bays. Also when I feel my SRI after a hard drive its always cool and I haven't experienced heat soak really. The pros of the ceramic header are that the engine bay will be significantly cooler. Cons include not lookin as nice as the SS header(imo), and durability is not as great although a lot of people have had good experience in that department as well.
The design of the header is much different than stock, it is a 4-2-1 design. The runners are designed as to streamline the exhaust pulses from the engine into a somewhat continuous stream and create low/high pressure areas. The low pressure areas will help "pull" exhaust gases away from the block, this is scavenging. Also the header has smooth long pipes that will reduce back pressure and allow the gases to escape more freely. You WILL lose a little low end because the runners on the aftermarket headers are wider than stock and at low rpms less exhaust gas is being pumped into the header. Smaller amounts of cooler of gas at lower speeds will expand quickly and fill the available space in the header, these gases will move slow and you will have back pressure. At higher rpms however this is not an issue and the system will flow very freely compared to with the stock manifold, you will have some serious gains in the top end. Overall your car will be faster, but in the low end you will have very very small losses.
As for emmissions they will be affected from what I've heard, but I don't live in a place where that matters. You'll need something that comes with the CARB sticker, I know DC has this but I'm not sure if any other companies have it. I have the OBX R header and I know it doesn't come with one.
EDIT: Just wanted to add my gas mileage went up with this setup (SRI+header). I can drive semi-agressively now and get the same mileage as driving conservatively before the mods.
The SRI and header is a good combo imo. The SRI gives low end and the header will give high end. Anyways sorry for the looong post but its late and I have nothing better to do really...
