I have been researching this myself when I noticed better efficiency with my Blichmann Floor burner, than my Bayou Classic KAB4. I am currently building my single tier.
My Blichmann is 2.75" from burner tips to kettle surface.
KAB4 is 4.75" from burner tips to kettle surface.
Purplehaze did a timed test on it already, so I don't plan on replicating. I think his #'s look good. And I believe Blichmann probably looked at that spacing for a reason. So I will be re-working my KAB4 to that burner distance.