Fascinating, I have flown with people as stubborn as this. Once they believe in an idea you can't confuse them with mere facts.
With your demonstration of climb and descent rates clean and with slats extended you would think that would convince him but I understand, some people can't be told or shown.
My own pet theory with this chap is his incorrectly perceiving the characteristic MD, almost vibration free and extremely smooth in flight characteristics with slats only extended as a 'no drag added' configuration.
I was very surprised, on transitioning to the MD80 from the B727 how smooth and vibration free the effect of extending slats only was.
But it doesn't mean there's no additional drag, and you conclusively proved this
