While you can control the temperature of a gas engine with mixture to some degree, cooling airflow through the cylinder fins is the main source of cooling. This can best be accomplished with proper baffling when needed. The BME 58 doesn't have an excess of cooling fins compared to other engines so baffling can be very important. This is not an alcohol burning engine where needle settings can have a major effect on temperatures.

Set the high speed mixture to where the engine runs the way it should (set just below or at peak RPM) and control temperature with airflow. Your engine will thank you in the long run.