What is your definition of peak performance if it's not measurable in terms of reliability or fuel economy? If the factory tune allowed for performance gains with octane we would expect to see it with increased mileage as is the case on the 2UZ. From what I can tell, that has not been demonstrated by those running 91 in the 3UR. So if the tune isn't taking advantage of the octane what peak performance is being missed with the 87?
Though see my comment on the last quote where I contradict myself a bit. So I will consent that if the 2 additional HP are because of the 91 octane then I would agree then that would be the peak. But that's like saying the top of the mountain is 87 octane, while 91 is standing on a step ladder and top of the mountain.

I say that in jest because it's not much of an increase. But the 383 on 91 does give credence to your argument that 91 is the ultimate in 3UR performance.
The marketing isn't the premium fuel, it's the 383 HP that the marketing house wanted. Imagine the conversation in the Toyota R&D lab: "Hey Bob, what if we put 91 octane in? What happens to the power? It goes up to 383? Perfect, we'll use that on the LX".
I tend to agree with this. At 91 octane in a lab they show 0.52% increase in HP (381 vs 383). Which lends credence to why we don't see any measurable fuel economy change. Assuming 12 mpg, this would be 0.0624 mpg change, not something you could measure. Though that also assumes a linear relationship between power and econ at partial throttle which doesn't happen in real life.