[Skoshi Tiger]

Of course EGT will only rise to the optimum Fuel air ratio and then start to decrease.

Google "lean of peak" and you will find quite a lot of arguments and discussions of the befits of "Lean of peak" opperation for enhanced fuel efficiency

Of cource this is all moot since the spitfire used an automatic mixture control and the pilot had limited control over the fuel air mixture. "Lean of peak" is more applicable to more modern airplaned with computer controlled direct fuel injection, ignition and engine monitoring.