Well, took the plunge and got the latest (offered) BIOS in. Still fairly hot, but while footling, found a performance setting on the S1 button with allows a change in the Thermal Control Strategy.
46 C on the output air and no change when selected to Performance mode.
I don't know if it's the main performance mode that's changed. I rather hoped it would be the fan control.
What I can't do is make that normal setting stay in performance mode - which can only be accessed via Control Panel and not Power for some reason. It never makes the Apply button come live and is back to balanced on the next look. Maybe Sony realised they'd made a dog, and stopped folk ramping it up to performance.