Maybe it's just me as I can't seem to leave anything stock. You might want to make it breathe a little easier with a cold air intake and then to re-tune the computer. You can make it a lot more fun to drive by having the right person set the parameters. Just changing the shift points should increase the fun factor quite a bit. The 2.92 rear gear can hardly smoke the tires on the GT although there is a lot of potential in that L76 engine.
I did the Vararam CAI, the heat soak relocation kit, Corsa exhaust and will probably install my Kook's long tube headers next week.
The G8 lacks a few interior touches like the homelink module to open your garage door. I purchased one on eBay for like $30 and installed it myself. The '09 models don't have a light in the glove box or center console.
The subwoofer amp is in the trunk next to the battery and you can turn the bass up some right on the amp. I reccommend this.
The 3-1/2" dash speaker level is too high and it's the worst speaker out of the 11 so you can either pull it out or put a resistor in line to attenuate it some.
The spare tire and jack kit for a Caddillac CTS will fit in the trunk well.
I really do think it's a lot of car for the money but having the LSX engine under the hood is such a temptation to tweak it for more power.
Check
www.GRRRR8.net and
www.g8board.com forums for a ton of info.