At John's suggestion, I checked the turbo bypass valve and found that it was still open (from rough pumping phase) -> Closed bypass valve -> OK

This is the first time that I have overlooked this in the "many" times that I have performed this procedure (there's a first time for everything!).  This proved (to me) to be a surprising explanation of the symptom since the bypass circuit is ~2' of 1.5" tubing (+ two 90s) connecting the turbo exhaust to the turbo inlet.  As such, I would have never guessed that a compression ration of ~500 could have been maintained with such a large conductance between exhaust-intake (who knew!) 

Also, isolated pump carts from BSC9 and BSC10 annulus volumes -> Both systems pumped only by ion pumps