I'm hoping for 430+ hp, and my guesstimate still stands at 437 hp. I don't think they will have any problem getting that amount of power from that 5.0L motor. One thing we need to keep in mind is that output will be limited by the gas mileage and emission targets (no gas guzzler tax and ULEV II). 431 hp is the minimum because IS-F engine revs at least 500 RPM higher than the 410 hp normal 5.0L V8.
410 hp * (7000 RPM / 6500rpm) = 442 hp
Even if the regular 5.0L V8 has only 400 hp, that would be
400 hp * (7000 RPM / 6500rpm) = 431 hp
If Lexus created something like the M3s gas guzzler engine satisfying only LEV II emissions, the IS-F could easily have 475+ hp.
Plus, given the ULEV II emissions, I would bet that an aftermarket exhaust could uncork even more power, no problem. I wouldn't be surprised if this one simple bolt-on added another 20+ hp, easily. I think the aftermarket will be very good for this car.