A very interesting thread to say the least!
When I was doing my commercial flying training in 1969 we were taught to "calibrate" the turn needle whilst on full panel before going to limited panel,ie do a rate one turn as indicated by the turn needle and time same and see whether this was correct. If not make an adjustment and check again etc. Quite often there was a small error - although I would not claim that my if/timing were that accurate!
Dan, very interesting to hear that you have had your a/c converted back to the "old" turn indicator. The turn coordinator also has a rate of roll element (as well as rate of yaw) due to the 30 degree tilt, etc. In theory should a wing drop the turn coordinator will show this before the turn needle will since the latter has to detect yaw before indicating. This is supposed to make it easier to fly limited panel (other than recovery from unusual positions) but I have yet to be convinced!