I have done some research on aftermarket exhausts for the 987. I don't think there are any really reliable numbers regarding the HP gains for making a change b/c there are many environmental factors etc... that impact the results for Dyno runs.
Generally speaking (directionally) adding just the exhaust won't get you anywhere near 30HP, more likely 10HP or less. If you add headers you will get better results, but still not close to 30HP.
Here are the main options I have looked at and some comments:
Fabspeed - nice quality but may cause some resonance. Depending on how aggressive you want the sound to be this exhaust can be pretty loud. For the 987 some don't like the way it looks from behind the car, the cans seem to hang lower (although they claim they don't)
AWE - heard very good things about this exhaust. High quality, more sporty sound, little to no resonance and not too loud. Shows HP gains.
Milltek - also nice quality with more conservative sound. Milltek never mixes the exhaust either, it is kept separate all the way through the tips. Usually costs $2000, but was on sale at suncoast for $1500 about a month ago. According to the Cayman forum the Milltek seems to produce the greatest HP increase, especially when using their headers.
Tubi - good quality, very good sound, but no known performance benefits. Also, I think this is spendy.
Just to reiterate, I was looking for the 987, so I'm not sure how this translates for a 986.
Based on my comments you can probably guess that I am leaning towards the Milltek, but won't be pulling the trigger just yet. If you want more info on relative HP gains and sound etc... Check out these links.
Milltek
http://www.caymanclub.net/reviews/showproduct.php?product=2&cat=4&limit=views&date=1161151595
Fabspeed
http://www.caymanclub.net/reviews/showproduct.php?product=8&cat=4&limit=views&date=1161151595
AWE
http://www.caymanclub.net/reviews/showproduct.php?product=3&cat=4&limit=views&date=1161151595
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