d series engines dont like bolt ons.. spend $500 on bolt ons and you got a couple horses. the exception would be the d16a1, its pretty different from the other d's.

now k and b series.. they look bolt ons.

CAI, header, test pipe, 2.25" cat-back piping and you might notice 15hp on a D15. next "bolt on" would be a race cam, which might give you a few more horses.. but beyond that your looking at building an engine, which you might as well swap in a better platform (b,h,or k series)