The difference is the collector mostly. A jdm 4-1 or a copy off it has a 2.5 inch collector, where a stock one has only a 2 inch. I just orderd a type-r copy in stainless steel which from dc sports is like $500 or something for only $200 shipped! (www.robearracing.com) Now the dc sport ones are carb legal, all except the type-r replica, and that goes for almost all headers i've looked at. A stock honda jdm 00 header goes for $450 and $70 for each heat shield. (there is 2) And that is the 2.5 inch collector everyone copies.

Now if you decide to go with this header you will still need to spend about $50 more, because it will not bolt up to your cat. You need a 2.5 inch flange and o-ring to make it fit on the cat which means welding is needed also. Not a stright bolt up item unless you already own a type-r.