This^ Parts off a Cobalt SS/Ion Redline or if you want turbo you can swap the parts off of an SS/TC or a Saab 9-3There's this about the Pontiac GXP.R Race Car: http://www.seriouswheels.com/cars/top-2008-Pontiac-G6-GXP-R.htm
But there really hasn't been much done to really push the envelope in these cars. Have you checked out www.G6Performance.com? You're more likely to find info there about heavy performance mods.
I know that there just isn't much available for the 3.5, but there is some, as you found. I know I've read about some users who are running cams, so I'm sure you can find more info.
I have the 2.4 4cyl in my G6, and my plans include supercharging and getting a nice bit of power from it, nothing over the top though... after all it is front wheel drive, haha.