I dunno... once you start getting into the upper echelons of performance vehicles, I can't see Lexus being able to stick around.
They still haven't managed to put a manual transmission in the IS350. I'm not saying that a manual transmission is strictly required, but it does show a certain amount of willingness on the part of an automaker to try to cater to the performance crowd.
In my relatively uneducated opinion, the IS500 will be an extremely competent, very fast car with an overbearing and intrusive collection of systems designed to keep you from doing anything fun. You know, like all of the cars that Lexus makes.