...so I've tried myselfs. 
I've re-written my proggie to first settle the PHC using EXTTS CH0
to the external reference PPS, and then switch EXTTS CH0
to the measured PPS signal (reconfigure the CH0
to take input from the second SDP input).
Guess what: the measured deviations are still quantised per 8 ns.

So to sum up:
during the initial settling of the PHC frequency / servo loop,
I can see offsets in low units of ns, not aligned in any way.
But: once the PHC settles to offset==0 && ppb==0,
any measured edges captured via EXTTS channel 0 or 1
will be quantised on 8ns boundaries.
This is kind of spooky :-)
Maybe the quantisation just doesn't jump at me so bad
while the frequency (ppb) is still a little off...
and it's true that if I keep capturing on both channels,
their mutual difference is always quantised at 8 ns,
even though the difference against the internal unsettled
PHC is not aligned that way.

I'm giving up this train of research.
Comments welcome :-)
8 ns granularity is better than nothing at all.

Frank
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