Looks like I have to throw in my €0.02 of comments!
The thing about the Lexus IS300 is the fact the car is very fast and handles quite well but the suspension is quite stiff and you will feel road irregularities quite easily. Installing the latest in low-profile tires in the 215/45R-17 size (Bridgestone Potenza S-03 Pole Position, Pirelli P Zero Assimetrico, Dunlop SP Sport 9000, Michelin Pilot Sport, Yokohama AVS Sport, Continental ContiSport Contact) may provide better handling but probably not a better ride.
I do think the Infiniti G35 will be good selling car--it offers a quite roomy interior, excellent performance and very good ride for a quite reasonable price.
As for the US-market version of the Skyline based off the G35 coupe--I've heard they're deciding between a turbocharged version of the current G35 V-6 or the 4.5-liter V-8 from the Q45 saloon. My guess is that the US version of the car will have the 4.5-liter V-8 but the engine will be tuned to nearly 400 bhp. I don't if Americans are ready to drive an Infiniti capable of keeping up with a Porsche 911.