Well, you can't really expect it to be light if it's a Lexus =p
It didn't really gain that much weight over the current IS300, nothing that the extra .5L won't take care of anyway.
The extra weight probably buys it better sound-proofing and better crash test ratings.
Anyways, all I have to say is that this is one fine specimen of a vehicle! I think it just crawled it's way onto my list of cars that I lust after...might have even completely displaced the BMW 3 series actually (save for the M3).
Of course the M5 is still solidly on that list but it's in a totally different price range (and thus, actual likelihood of owning range).