Here is a consolidated graph showing a variety of mods. There is a baseline plot and then each plot shows the compound effect of the mods. Don't focus on the ultimate number instead look at the shape and gradient of the plots.
The car in question was my 2000 S. UK car, so only 2 cats. Baseline is at approx 60k miles and the other runs are between 60-70k. All runs done on the same dyno. The dyno operator has no axe to grind. He does not tune or modify cars as a business. I have never bought anything from him, not told him what mods have been done to the car. This is the UK remember so the weather and temperature is pretty much the same all the time!
http://986forum.com/forums/performance-technical-chat/47555-tuned-modified-2000-boxster-s.html
The plot below is for my 2004 550 Anni. Again a UK car so no cats in manifolds/headers. Same dyno as above. Approx 105 miles. This car has a K&N filter (987 airbox as standard), ebay stainless manifolds/headers, Carnewal exhaust (internal modification only), 100 cell cats (ceramic I think). It also has a 4" underdrive pulley but I doubt that adds much if anything.
This estimated flywheel figure is 272bhp and the plot for wheel horsepower shows 223bhp which is consistent with Jake Raby's above data of 18-22% drive train losses.
Somebody correct me if I am wrong but the AFR here is pretty much there or thereabouts so a "tune" in my opinion is not going to see any significant gains, certainly not to trouble 300bhp.
On the same dyno on the same day in the same conditions my car made 10bhp more than 2 other standard 550 Anniversary cars.
I used to think optimistically that if I had a strong base standard car to start with that I might crack 300bhp but it's just not going to happen at the same rpm. People often compare to a BMW M3 but forget that the limiter and where the power is made is much higher. If you extrapolated my 272 plot from just under 6500rpm on the same gradient and by another 1500rpm to where the M3 makes its power then the peak number won't be that much different.
I have done the throttle body mod on my previous car (above), and based on the gains there, you can see they are not huge, that mod will give me a bit more but not 10s of bhp.
There is a chap here in the UK I know of who has an early 3.2 987 and has experimented with various bolt-on mods too and has not quite cracked 300bhp either. That cars starts out with 280bhp ish as standard.
There is another chap who is a tuner and engine builder by trade, and a quite distinguished one at that, but who is new to Porsches. He saw a good gain on his rebuilt 2.7 by a slight increase in compression from a head skim.
I'm no trading expert but just an enthusiast who has wasted many £££s and hours mucking about with a toy car. Sure i could spend more money on supposedly better branded items and maybe make a little more power but that is not what I am trying to do. I am trying to experiment on a deliberately small budget. There are lots of butt dyno claims but the only real test is as above. I also felt that when I had done the manifolds/headers and exhauston my first Boxster that the butt dyno gave me a lot more than the real dyno.