I believe the shiny aluminum was reflecting infrared radiation back into the engine bay like a mirror. I experienced this with the aluminum flashing I had on the previous engine belly pan. All the black paint allowed was the absorption of the radiation and the unpainted side of the diamond plate disperses the heat into the airflow.
Not looking to do an ABA testing session. The closest experiment is fer others understand is to allow the sun to relect off of some diamond plate onto their face, cover the diamond plate with a sheet of newspaper, and then take the newspaper off.
I've never had front wheel skirts before. Perhaps you have my rig mixed up with someone elses? I'm not understanding the wheelwell question...