welcome to the forum 2000986s 
I also have a 2000S and made a few air flow upgrades. Fabspeed sport headers/cat, Fabspeed secondary cat deletes, and a Jetex muffler. The sound is a huge improvement, as for the horsepower gains...slightly noticable, but not worth the capita invested(if you were going for the HP gains). I do not regret the upgrades though, because the glorious sound it makes now, Justifies my exhaust expense 
I'd recommend doing some research of a few exhausts before pulling the trigger on any upgrades. With that said, +1 Need_for_speed for the exhaust breakdowns!
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2000 Boxster S, SPEC stage 1 clutch, Fabspeed sport headers, Fabspeed secondary cat deletes, Billy&Boat muffler, EVOM air intake, Ben006's Custom Short Shifter, Pedro's TechnoBrace, Eibach rear sway bar, De-snorkeled, Fred's custom projector HL
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