I work at an engineering college and one of the professors is a fuel expert. So, I asked him the question about octane, compression and knock. Here is the engineerese translated into plain english.
(Disclaimer: It is your car, you do with it how you feel is right...)

Octane controls the temperature where the fuel self ignites. The higher the octane the higher the temperature. There is more to it than this, but this is generally how it works.
In the winter, the refineries up the octane of all the fuels. This is to maintain engine performance and emissions.

Compression causes heat. If the compression of an engine is high enough and the octane of the fuel is low enough the fuel will ignite before the spark and this causes knock. The NA Crossfire has a high compression engine so higher octane is a must.

In the hot summer, the intake air is already warm, then it is compressed and even hotter. A low octane fuel (87) would probably knock and give the car fits even though you won't hear it. That is why in the summer you have to run 91+ octane fuel in the XF. In an emergency, 89 would work as long as you drove conservatively. If you drive in a spirited fashion, 91+ is the only way.

In the winter, the intake air is much cooler (well, in my neck of the woods) so the cylinder will be cooler during compression. A winter 89 octane fuel is probably closer to 91. An average driver could run 89 octane fuel in the winter without any problems. An advantage to using 89 fuel is it will start much better when the engine is cold. Again, lower octane lower temperature for ignition.

The SRT folks should run 91+ all the time because they are double compressing the air. Sorry people, hot car, hot fuel!!!

This is my 2 cents...
Mark