I've been in a DC5 for years now and over that time the car has been in various states of tune.
Looking back I think I've had maybe 5 x different cat-back exhausts, 2 x inlet manifolds, 2 x exhaust manifolds, 2 x induction setups and even 2 x turbo kits oh and a kpro
So to answer the original question, with £/per bhp in mind on a naturally aspirated integra I'd say stick with the stock airbox.
I've also done a couple of comparison tests before and after with various "performance" mods and the same result, although the figures on a dyno might be different by as much as 10hp, in the real world. There was no real difference in performance on the road.
My conclusion is that in order to make any sizeable difference in actual "acceleration", you need a hell of a lot more HP (like 50hp minimum for example) to even notice the difference and we all know the easiest way to do that is to force the induction with boost.
I would argue that the difference between a 220hp intergra and a 250hp integra would be so marginal on the road that its an absolute waste of money when you think what you have to pay to get your bhp there.
As others have said its all about the sensation and driving experience. So if you want noise and theater by all means get your mods on and choose them to suit your preference but if you are looking for £/per hp leave it stock and get a final drive installed.
Or get your wallet out and boost it
my 2p