Its a myth that the supermarkets provide inferior fuel. All fuels in the UK must be within a very high standard. There is a large marketing drive with pro fuel companies (shell, esso, etc) to say theirs is much better. Its not all that better. Cars that are built in the uk are designed to run on 95 octane. Higher octane fuels that are available are not much use to cars not ecu programmed to run on such fuel cannot take advantage. Any improvements that it theoretically can give is compensated to bring the car engine performance back to standard.
With the Lexus, its the same as if you put on a larger exhaust, intake or manifold, etc. The car will enjoy small benefit for a short period, compensate then bring all gains back to stock.
I have run many cars (Impreza, RX7, Skyline, etc) on standard supermarket fuels, and have never had any issues - performance or otherwise. I have also got the Skyline dyno'd a few times with different fuels and found negligible differences. Mainly because the ECU wasn't programmed accordingly.
Remember, alot of these fuels are actually the same - same refinery, etc, but with different labels. This happens with many many many products.
PS - Apologies for spelling errors!