This may not go down well but I fundamentally disagree with there being little difference in premium fuels.
Porsche engines are, fundamentally, quite highly developed pieces of kit and have been tuned to be efficient and to produce their power at certain compression levels. The internals of an ICE are basically: inlet, compression, ignition and exhaust. Air in, squeeze, bang, exhaust. The problem with petrol (ie gasoline) is that high compression can lead to premature ignition. That is what the octane is there for - previously, leaded fuel was used to retard ignition, now you have some tricky chemistry that is less lethal! The higher the octane, the more premature the ignition, the higher the compression and therefore the more efficiency in the engine. This then leads to higher/more efficient performance. This will apply equally to performance diesels.
If there is premature ignition, the engine's knock sensors kick in and retard performance - basically to stop the engine from rattling itself to pieces. Granted it may not happen immediately but over time this causes undue wear and tear on the engine - or so I am led to believe.
Anyway, that is essentially my view. I will always aim to put the higher octane stuff in. Obviously if you're banging around in an everyday rep mobile there isn't a need for the higher octane. You can use it, but it's wasted on the car.
Porsche, in its manuals, always recommend using the higher octane stuff. This is primarily why.
G
PS...sorry...I'm still using V Power!