I've built several turbocharged I6s using the early Buick turbocharger. With water injection and a modified boost control I saw 16#s boost drawing through a single barrel. I'd hazard a guess that would be in the 300hp neighborhood. That engine in a '64 Nova ran 102-103mph through the quarter, but suffered a lot from wheelspin so the ETs stayed in the low 14s.
I've got another one sitting on the bench now ready to go if I find time. I could e-mail a pic of it if you're interested.
