ORIGINAL: Cyberwolf
Any engine will cool better in the air, And again if your engine is set where its supposed to be you won't have a lean run in the air.
An engine may or may not run cooler in the air than on the bench. Factors like cowlings and the aerodynamics of the nose play a major role. You are correct that the engine needs to be set "where it's supposed to be". Far easier and safer to determine this setting on the bench, especially if you have duplicated tank size and location, fuel line length etc. as will be used in the plane.
jess