Because the stock system rusted out, I put a DC Sports header and Apexi Word Sport cat-back system on my old '92 Accord EX. There was a rather significant difference between before/after. Might have had something to do with the stock system being rusted out, but I never remembered the car having that much power before.
Anyway, my butt-dyno told me there was a solid 10 to 15HP gain in there on an 140HP car. I never did the intake because, well, I wasn't modifying the car for better performance, just replacing broken parts with aftermarket ones.
Some engines respond well to bolt-on modifications and some do not. Based on what I've seen of our 2.3L, I am hopeful that there is some room for improvement.