It would have helped if I said brake servo well worth doing and was a mod carried out by the MOD.
Most owners seem to want to keep the vehicle in the spec that it was used during service any mods r usually stop gaps till they find the correct parts. Any mods that they do find to be of use end up being hidden.
Not the answer you're after but you have already mentioned the basic ones battery cut off switch and the like.
I'll add another and I know Ian Wallace had this idea and did it, LED replacement lights for the side, indicator and brake lights. Indicator lights required more resistance in the circuit for them to operate the flasher unit.