Klunk:
So, with this in mind, why can't the routers firmware pick up the fact
that the statistics on the line have changed, and then refresh the sync?
Surely this is a pretty basic thing to do in router firmware if the
vendors wanted to do it?

Am I missing something?

Yes, the BT DSLAM will NO longer play ball and just sticks to min SNR
prescribed by BRAS for that line.
Therefore I think that routers that can set their own min SNR will be of no
help.
Regards,
Martin