The USA and Canada rate octane using (RON + MON)/2. MON tends to run 8-10 points below RON depending on the composition of the gas. In Europe and Australia octane is rated by RON alone. 91 octane pump gas in the USA and Canada is virtually equivalent to 95 octane pump gas in Europe and Australia (do the math). Your F800GS will run just fine on 91 octane premium unleaded fuel as delivered without the need for any remapping.
In Mexico I have had parts ping at me when I washed them in Mexican gas, so you are taking your chances down there and in many parts of South America.
my favorite bike - R1200GS