Hopefully I can shed a bit of light on this... Back in the day, I did some durability testing on the S54 engine and from the factory it had one of the best adaptive strategies for controlling ignition by using knock sensors - it really could throw in and drag out spark timing with varying fuel quality. So, in theory, you could see a difference in performance from high octane fuels.
However, a good fuel isn't just about octane, it's also about heating value or the energy in the fuel along with a whole set of other parameters. All UK fuels have to comply with standards which have nominal values for octane and heating value and there are also standards for some physical values depending on time of year so that cars start in winter and summer. So, some fuels might have a high octane rating and (relatively) low heating value by using ethanol as an octane booster - great for octane but low in heating value. Others might use aromatic components to achieve high octane AND high heating value, but that also depends on the supplying refinery.
Also, most fuel manufacturers will have their own additive pack to make their fuels different - Shell does, so do BP. These are detergents usually (to keep injectors clean) and other things like potassium to protect exhaust valves.
Trouble is that most fuels need a good few tankfuls to feel the full effect with some engines responding better than others.
Truth is the S54 likes octane - it's a high performance engine so I'll be buying 98 Octane whenever I can from a major supplier to hopefully keep injectors and inlet valves clean...! 95 RON isn't going to do any harm at all as the engine management will adapt.
Interestingly, formula one engines (V10 and V8s that were normally aspirated) didn't need high octane as their engine speed was so fast (18000+++) in 2006 that detonation didn't have time to happen - heating value was the dominant factor there.
