i've never been entirely sure of how a split image focusing
dealey-bopper in an slr works -- is it dependent on separation
like a rangefinder? Something else?  I do know that there's no
substitute for a good rangefinder, especially in dim light.
Leica said its rangefinder was so accurate that if you put two
pins, one inch in front of the other, a meter away and focused
on the one in front you would notice a separation of the images
on the one in back.  With an slr both pins would look the same,
but I've tried that with a rangefinder and it works. With a very
narrow depth of field, that error can be critical.  So, long way
of saying -- if I were you I'd assume the ground glass is
accurate, not the split image.  ps...i see some people already
tried this with an slr and say they can see the difference
there, too -- ok fine but i bet its a lot quicker in dim light
with a rangefinder