That's not quite the full picture. The likelihood of hot spots which act as glow plugs is a factor. If the combustion chamber has sharp or small features that stay hot from one compression stroke to the next you will need higher octane to avoid pre-ignition regardless of compression. Also, from what I find there is not an exact relationship between compression ratio and octane requirement. An engine will tolerate a range of octanes for a particular compression ratio. So my assessment is there are two main factors in why those of us who do so, successfully use 87 octane in our Spyders with no adverse effects. First, the combustion chamber is a very smoothly finished with no sharp features that stay hot from one cycle to the next. The second is the engine design and compression ratio are such that it tolerates 87 quite well, but 87 may very well be at or near the minimum octane required. 91 octane probably falls near the midpoint of the best octane range for our Rotax engines.
My experience from 2016 and 2017 does not support the argument that ignition timing is retarded, and so power is reduced, when we use 87 octane in our Spyder engines. If you look closely at the graphs in my thread linked to in post #28 above, you will see I had a very slightly better mpg when using 87 octane.
Now, an interesting question came to mind while I was looking for info about compression ratio vs octane required. Ethanol is an octane booster. I believe most all of us, if not all, who regularly use 87 octane use E10. Has anyone here used 87 octane ethanol free gas? If so, have you experienced any pre-ignition issues at all? I wonder if ethanol is what makes it possible for us to use 87 octane without problems.
What we really need is a comparison of the combustion chamber of two engines with 50,000+ miles on them, one having run 87 its whole life and the other 91 its whole life. Then we would have objective data to support or discount the use of 87 octane.