Getting rid of the old style single downpipe manifold is a good thing. Putting a slightly larger exhaust on back to the tail pipe is a good thing (although a stock B18 probably doesn't need too large - 1 3/4 perhaps?). A 4-into-1 header will be an improvement over the single downpipe manifold, but not really any better than a much cheaper stock dual downpipe and matching manifold.

And even if you do have a tiny exhaust system and a single downpipe manifold you won't notice much of a difference below 4000 rpms or so. But from there on up to the redline it will pull harder with a more open exhaust. Swapping the cam (assuming you have a 'C' geind') for a 'D' grind will help you in taking advantage of the improved breathing.
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I'm JohnMc, and I approved this message.