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the car already has a cold air intake and doesn't work well with oiled filters (the reusable types like k&n, afe, etc.) so not much to do on the intake side that isn't relatively intrusive. if you are handy search 987 airbox on this forum (we swap our airbox with one from a later generation boxster).
exhaust is difficult also. the most gains can be found in header design (vs just catback) but changing headers is difficult with the catalytic converters - replace oem headers with aftermarket catless headers and you have to deal with check engine lights (cels) emmissions issues, etc. if you do go that route, there is some inexpensive stuff out there (chinese knock-offs of the high end guys like borla) that you can get for 1/10th the cost. also search this forum for ways to deal with the cel issues.
the best bang for buck is an underdrive pulley (udp). it's a smaller diameter drive pulley that puts less parasitic load on the engine and is good for 5 hp or so. note there are different sizes - go for the smallest (4" if possible). tarett is a good source and a good seller. i think pelican parts (owner of this forum) also sells them.
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