I drove one for 3 years up to ~105k miles. Not sure where the "maintenance is costly" comment is coming from. While I was still working at Belle Tire, I replaced a wheeler bearing and a ball joint. They're no more expensive than any other car..? The wheel bearing was available same day, but the ball joint had to be ordered. Some of the vendors on the two main G8 forums are able to give substantial discounts on almost any part you could possible need. Because the engine is an LS variant, parts are typically readily available if the need comes up, although I never had a single issue related to that. I actually never had an issue period.
I had Kooks LT headers with mid pipes to stock exhaust, Rotofab intake and a Livernois tune. It went 13.0 @ 108 and I daily drove it throughout the year. I put winter tires on it for the cold months and never had an issue. It is a heavy car, but I wouldn't ever call it slow. I'm not sure what NBFRC is comparing it to, but in stock form it's at least not bad. With a basic intake and tune you're in the very low 13's, which is fine for a large sedan.
I do agree the prices tend to be high. You won't find a GXP for less than $26k - $27k, and some of the sellers are asking more than what they were new. I really don't get who would buy a G8 GXP for roughly the same price of a new Chevy SS, but I've seen more than one priced that way. The GT model is certainly better than that, but the low mile ones are still getting $20k plus.