I did Euro delivery and my 1M felt faster in Europe but not sure if it was because it was new to me. With that said I believe Europe and US use different rating methods so it's not equivalent. Performance would drop if the car detects pinging but 91 should be enough to prevent it unless you got bad gas. I don't believe 91 or European 94 would make that much difference unless you were in high heat or track where detonation can increase. I have owned two 1Ms and have driven them in Europe, Canada and the US. Even with 91 octane I have always found the power delivery in the 1M to be slightly unpredicable. On cold morning, it is a monster and warmer days it's more tame. I have gotten pretty used to the power but every now and then the universe aligns with the right 2nd gear, rev range, temperature and who knows what else where the car catches me offguard and launches like a 400hp beast. It has happened to me a few times where everything is right. The traction control doesn't bog down the tires are up to temp and motor is in some sweet spot.
I don't think the difference is performance is 91 vs 94 euro or higher. I think the nature of these motors coupled with traction control, temperature makes the delivery vary slightly.
With that said, bad gas that has been diluted can reduce performance but once you are in the 91 US range, you are fine and more octane will not due anything unless on the track or death valley in summer where detonation can happen due to heat issues.
I have heard that gas stations order a set amount of regular unleaded and if the their underground tank fills up, the dump the remainder into the premium tank since they bought it already. Europe has more consumer controls than US so it might happen less there but that is possibility here.
That's my two cents worth.
Here is article from The Economist on octane. They state that 94 euro is equivalent to US 91 . Also that the US standard is more comprehensive than the RON Europe one.
https://www.economist.com/babbage/20...-needs-premium