I think someone touched on this earlier on the thread, but Lexus would have set a target figure for the ISF when they built it. The engine, intake, cam, exhaust, ECU etc would have been chosen to achieve that target output. The simple fact is the engine can produce closer to 500 if you add different parts.
Yes it could alter the low and mid range figures, but it will gain in the higher range and produce more power.
I've not started digging into aftermarket ECU's yet, but if something like the SYVECS we used on the Supra will work, Ryan at 2bar tuning can re-map the car to achieve an optimum setup with all of the extra components.
Look at the GTR. it's a well known fact that no two GTR's produce the same horsepower. Some got more than others, despite having identical blocks, heads, cranks, cams etc.
Lexus also play the long game, and may have held back the ISF, so they could add power to the RCF.
Either way there are good gains to be had if you have deep enough pockets, and a desire to gain more power. Same applies to most cars, but turbo'd and supercharged cars are generally easier to do it with than n/a's.