" Either that or the prop unloads a bunch in the air as the plane seems faster and louder (faster reving) in the air. "


Rob,
Actually I'm not surprised at the PB prop loading the engine more than the MAS under static conditions as its a very efficient design and thin blade section(thickness) will allow it to bite better than the thicker MAS paddle, but under flying conditions the thiner section should allow it to windup better due to its lower drag as it approach's its pitch speed.
A good demonstration that static RPM isnt the beall and endall of performance.
I have just builtup a Norvel AME 061 useing a new chrome type piston/cyl set which has little taper and a great piston fit , RJL diesel head and shows real diesel snapover( i bought 4 piston/cylinder sets and only the 1 set has this fit, the others are typical norvel fits), I think the PB 6x4 should work well on this too.
Stewart