The 1st Generation GM High Feature V6 is a fine engine, not a powerhouse but does make good HP. They really don't give better MPG in the G8's over the 6.0L V8.
The V8 is hooked up to a 6 speed automatic, the 3.6 V6 is hooked up to a 5-speed Automatic. The V8 also deactivates 2 cylinders when cruising.