[Reiver]

If your speedo isn't working properly it is hard to get a proper figure and my speedo and odometer are off 6 mph over or 60 miles every 1000 clocked.
I'd change to the VR1...you need a good flat tappet oil anyways (see some of the oil threads about zinc/phos etc.)...and do a route drive of a known distance at normal throttle/rpm and see what you find out.
300 mpq is definitely rebuild time for the top end (you didn't say what the overall mileage was) and in the 600's is rebuild time for me too altho some keep at it.
In my old 930-16 (83 SC US model) I'd get in the 600's mpq and only see a puff of smoke on start up and hard deceleration. Adding that much oil is a pita.