IMO they need to be stuffed like Nick's ride.
My last IS had 18's without a drop and honestly looked pretty stupid.... but that one was the AWD and I read a lot of people getting bad vibrations from dropping the AWD.
The one above is dropped 1.7" and the 19's fit perfectly without a rub.... its amazing what a little drop will do, especially when I park next to a "stock" IS350... but this obviously isn't a tow vehicle, so a drop is a little easier to justify. I'm suprised that even on 30 series tires the ride is fantastic with these newer springs.
... but its about that time, snow tires.