While I'm not in a position to argue all the physics discussed here, and I can't prove or disprove claims made by people either here or in other forums, claims of 2X stock horsepower for my donor engine with little more than an aftermarket turbocharger and intercooler kit are common in Mustang circles. Yes, everyone could be lying, but I don't think they're are all liars. There's just too many of them.
Wanting to illustrate the point (big HP gains from turbocharging), but not wanting to bore the pants off everyone by asking them to read many forum entries elsewhere (as I have), here are a few videos of dyno tests for the Ford 3.8 V6 turbocharged in several configurations. Not all are completely stock, one uses the 4.3L truck crank, but most are 3.8L and in street cars. Given the relationship of the engines, results from Buick 3.8L V6s might be a good substitute.
I don't expect these videos to convince everyone that the results are valid. I'm not claiming they are, or that videos make it so. But at least introduces the idea that these claims may be plausible and could even be truthful. Besides, it's more fun than looking at graphs! From what I've read, the stock 3.8 Mustang would dyno at around 150 RWHP (Rear Wheel HorsePower).
1) Claim: 309 RWHP, 2000 3.8L V6, stock internals, 10 PSI
Verification: None really, results of dyno test at end of clip
Sorry for the music. They do it because they can - suxxhttp://www.youtube.com/watch?v=SmdYiP6QYbw
2) Claim: 474 RWHP, stock internals, No pressure given
Verification: Dyno screen shot, Verbal, multiple runshttp://www.youtube.com/watch?v=pMEkc0DD2Fwhttp://www.youtube.com/watch?v=YjHnVodWKVM
3) Claim: 476, 557, 611, 579 RWHP, Twin turbos at Various Pressures for Modified 4.3L (Truck) version
Verification: Quick dyno screen shots, multiple runshttp://www.youtube.com/watch?v=OgqkOemJeQE
4) Claim: 434 RWHP, 3.8L V6, Modified internals, ~14 PSI
Verification: Dyno screen shot, Verbalhttp://www.youtube.com/watch?v=0HWYq-WAGPU
5) Claim: 540 RWHP 3.8L V6, Modified internals, No PSI given
Verification: None what ever, could pie in the sky, but fun to listen to anywayhttp://www.youtube.com/watch?NR=1&v=Vj2SrCskyoY