Yeah, insignificant. Octane rating is a derived number that quantifies a given gasoline blend’s ability to be compressed in an air mixture without detonating; it doesn’t (normally) relate to the energy content of the fuel. /pedantic
To be even more pedantic (

On another note, and yes this isn't a winter result, I have just returned from up the coast. 115km on the trip, 54% battery remaining, estimated range 250km on the nose.This was a combination of 90-110kph with a small section at 80kph (road works) and a few km of city (at the end). In my case, the aircon was keeping a nice 21C on a 30 plus (more like 35C) outside day.
As all things here are upside down, my summer range is worse than winter as I use the aircon in summer. I think cooling is more efficient than heating in the MINI, especially when the resistive element kicks in.