Minored on combustion fueks for my Masters.

Simplifief somewhat, the higher octane fuel actually has a lower calorific value than the 95 RON petrol, so unit for unit is actually less energetic. A few cars, modern Golf GTi's, Evo and Scoobys, high performance turbos, can advance their timing and thus increase their power output, but theyre not so much gaining any power as simply operating as intended in tje first pkace. More conventional engines will see zero performance improvement - it would be against the laws of thermodynamics to do so.

The diesel is a different kettle of fish. Diesel can not advance the ignition (although Mazda are about to launch a hybrid spark ignition/diesel unit that can) , so questions over octane don't apply. The big difference here is the additive package, which may clean the fuel system and engine internals and not so much increase power, but maintain it at its optimum.

I've yet to see any scientifically valid testing from an independent source that shows an improvement in economy or power for either of these fuels. Keep your engine in good fettle, drive it properly (particularly pertinent to diseases) and they are of no benefit.