Well, if you put 2 options: 'on startup, switch to X mode' and 'on exit, switch to Y mode' (by default, probably set performance on startup and return to the default, ondemand, on exit). I think that should cover most use cases... if it crashes now it leaves you running in the state it was, so it wouldn't be any different from that. Can anyone think of a counter-example? Does any other piece of software attempt to change this as well?

Cheers.