Originally posted by MrFatbooty
Just remember that you get no benefit from the 2.5" collector unless you redo the whole exhaust system to 2.5". That means replacing the cat with a 2.5" high flow one, and having custom 2.5" exhaust pipes bent back to a muffler (preferably with a 2.5" inlet).
I agree with replacing the cat to a high flow, but I disagree that a 2.5 catback exhaust needs to be used (though it wouldn't be a bad idea) and here's why: the exit on the JDM collector isn't really 2.5... it's more like 2.25 and the A-spec header exit is around 1.75. So using a 2.25" cat back really doesn't create a bottle neck like you'd think it might.
I highly recommend going with a J-spec header. It will give you a signifigant power increase, while like snoopy said in his first post, an A-spec header wil only net 2-5 hp.
To answer your question of which header is best, HyTech and SMSP are. And yes, they are worth the wait and money.