Listening to all you guys and Joel's words on rim widths and profiles, I went and took a long hard look at the BT 090's. The rear profile on the rim is such that I was able to use and wear the complete tire from centre up to the outermost edge, without ever feeling like the bike even thought about sliding. The width on the rim just clears the chain and gives the look of a 150, due to the flatter profile.
The front is well shaped and evenly worn, but still plenty of life left. Maybe a fatter front at a later stage?
So the clever thing to do is to just replace the rear BT 090 again, as the front is still good.
This is a racing combo which grips well in all conditions and is only available in that size, for the WR 250 track bike. So I may have to live with the lower mileage!