The G8 V6 came stock with 18 inch wheels, not 17. However, somebody may have put aftermarket 17 inch wheels on your car.
I have a hundred pounds of junk (overnight bag, spare tire, tools, etc) in the back of my G8 GT and that seems to be the right amount of weight in the trunk. Adding another 40 pounds didn't improve it noticeably.
Tires are the most important thing. My G8 came with the 19 inch wheels and the summer-only tires, and they weren't kidding about the summer-only part. I bought a set of used OEM 18 inch wheels for winter but the attached stock Goodyear RSA tires were horrible. A week later I bought Michelin Pilot Sport AS Plus all-season tires, which are good in southern Ohio but might not be good enough for Wisconsin snow.
Gone but not forgotten: 2009 Pontiac G8 GT, White Hot, Premium, Sport. Tune is "Down Under" by Men At Work.