The July/August Issue of Car&Driver has a four car comparison of the IS500 F Sport against the Acura Integra Type S, Audi S3, and the Cadillac CT4-V (not a Blackwing!). It's a very good read. The IS was the fastest, but the Audi showed the same times (probably because it's 400 lbs lighter and the IS is a pig), and it placed last in the group over all.
That was essentially due to it being the oldest, heaviest, and most expensive. It placed highest in Driver Comfort, Engine, and Ride. It placed last in Ergonomics, Exterior Styling, Fuel Economy, and Brake Feel. But I believe that if cost were not involved it would be the one that the testers would own.

They said that the IS350 F Sport was closer to the 4 cylinder turbos (meaning that it would have lost the comparison, too) but that they fell prey to the siren song of a 5L NA V8 so they picked the IS500 for the article instead.

Peace