Im interested in bolting on a set of DC Headers on my 2007 civic si. Only problem is I live in Southern California where they are beyond strict with emissions.  Is there any way to tune a header back exhaust to where I can still meet CARB requirements?  If not, what can I do to maximize my exhaust flow with my current magnaflow cat back?  

if you touch anything that has to do with emissions, your car will not be legal.  if you run a full exhaust system with high flow catalytic converters, you will probably pass the sniffer test (actual emissions testing), but will fail the visual inspection for having something other than the stock emission components.

running catless on a civic Si will not give you significant gains, not enough to risk the state ref trip.  Cars with stock forced induction benefit heavily from running catless, but NA motors dont see a lot of power gains.  Not sure if they make headers that will allow you to retain the stock cats, but I would look into it.

of course you can always just run a straight catless exhaust and hope you dont get caught, but trust me on this one personally, the state ref is no fun    

Yeah the state ref is not a place I wanna go.  My friend has to make two trips to the ref in down town LA for the same ticket. One for emissions, and the other for DB levels.  I know AEM and DC make 4-2-1 headers for my car.  DC specifies that the cat is eliminated, but AEM doesnt mention legality with their product.  Do you think custom headers to retain the cat would be a reasonable idea if i saved up my cash for a while?  

You could get an old (anything before 1972, everything else is severly choked up from the factory after '72) detroit muscle car and never would have to worry about emissions testing. You still have to worry about the db level, but you can pump as much noxious gas as you want into the atmosphere. Since this is the Import Tuner forum, I will not dare suggest a thing. A good majority of the tuner cars are not emissions legal, they are just changed back to stock before testing. My friend's Civic failed countless times until he just put the stock system back on. My honest suggestion is to find someone licensed to do emisions tests away from the usual outlets and have them certify it. It's hard, because they are at risk of losing their licenses and jobs if they do, but they are out there. Also, as long as you aren't reving your engine like a madman at a stop, then you should be at no risk of getting a ticket for db limit, pending the cops aren't *** and you have nothing done to your car visually that would warrant you being pulled over in the first place.