I thought that the US's "premium" was basically equivalent to our standard grade petrol, and that they also had a lower grade on sale.
Yes that's correct, their 91 premium is our standard 95 RON. They do also sell 92 PON in the US but also lower grades that haven't been available in the UK for many years now.
With the lower grade fuel in the US, why are many of the Lexus engines tuned for more BHP than in the UK Not much different on the IS300, 215 compared to 211 but on the old GS430 there was 20bhp difference.