I just got word that Hahn Racecraft just made 408 hp at the wheels on 93 octane pump gas and 422 hp on 100 octane (with more tuning to go), using its bolt-on upgrades on a stock motor for the 2.0L turbocharged Cobalt SS. Check out Hahn's website for info on this build as well as its catalogue of intercooler, turbo, charge piping, tuning, downpipe, intake, and exhaust upgrades.
If you are as turbo-happy and boost-crazed as most of the people I know, than perhaps you have heard of Hahn Racecraft. A few years back they built a jaw-dropping twin turbo kit for the GTO that was displayed at SEMA, and since then have been knee deep in Ecotecs including Rhys Millen's Red Bull-sponsored Solstice drifter. Hahn is also quite well known for its Hayabusa turbo kits, which leads me to believe Bill Hahn (CEO and Head of Engineering) could possibly be off his rocker.