to be honest the 17/25 rating isn't what kills me on the v6 with the g8. hear me out, i've driven the v6 and v8 of the g8. i've driven the aura xr with the 3.6 and the thing has balls. the same 3.6 in the g8 is neutered and i don't think its due to extra weight per se. its the $h!ty transmission they put in the g8 v6. the 5 speed auto i did not think harnesses all the power the v6 has. plus all the power robbing 90 degree changes in the load path of the driveline that rwd has vs fwd. if the v6 became like a 290hp DI and the tranny became a good 6 speed (even ford has good 6 speeds now) then i think you'd have a scenario where the v6 is desirable and could conceivably justify ownership in spite of its gas mileage. keep in mind many fancy imports with 4 poppers and such don't really get better mileage either in ratings or real world. the small block at low rpms will sip fuel on road trips but will guzzle it in town which is where most people will drive. I am tempted by the notion of a stick shift GT but the RWD only makes me nervous in my climate, even with snow tires (all that power).
even though pontiac touts 50/50 weight on the G8 GT if you read road tests its coming in around 52-53% on the front which every percent hurts you in a cold climate as far as rear traction. the turbo 6 might weigh as much as a small block though, and even the largest 4 popper turbo which would be light on the front end might still not have enough torque....hard to say.
in all my rwd cars, the ones that were worst in winter and in general for nose plow were the heavy v8 cars. my 77 century coupe with the 3.8 231 v6 (like 100hp!) was so light on the front end in comparison to the v8 versions of the regal and cutlass of that day, but it was one of the best winter cars I have had. Neutral in the snow because the front was not a boat anchor.
ultimate point.....i don't think the gas mileage for the v6 is that much of a transgression when really its the fact that the powertrain does not utilize the gas or the 256hp (whatever it is) in a very efficient and smooth fashion. By my seat of the pants the v6 g8 is a full second or a bit more slower to 60 than the Aura XR which to me is troublesome. Notice we don't have instrumented testing of the v6 g8 in the buff books yet. Anyone knwo what's its testing out in AU? final point, the aussies get the v6 stick combo......wish we could.