(Apologies for resurrecting this old thread, but I thought that my somewhat delayed conclusions may help others too).
So, I waited, ... and waited ... and waited ...
and nothing happened!
Following the change of my service level from 'Super Stable' to 'Stable' my target SNR reduced down to 9 and I stayed there for ever. Then months later with no target SNR auto-reduction I gave up and contacted my ISP once more. I asked them to change my 'Stable' setting down to to 'Normal'. Well, within 14 days I received the automatic resync from the exchange and suddenly I found myself running at 6dB SNRM and sync'ing at a full 7616Kb/s, which gives me a BRAS ADSL 6500.
The error count has gone ballistic (see graph) at times higher than what I had previously experienced during lightning storms! However, with the new setting of 'Normal' the line seems to have been stable, not losing its sync at all. This is despite some line cross talk which significantly increases the error count during 4-5 days a week. I have been running now for more than a month and have a stable connection and the highest sync speed that my 20CN connection will support with Interleaving on.
So, having been around the houses I have concluded that the first thing to tweak for achieving and holding onto higher sync speeds is the service stability level. 'Super Stable' requires a very quiet line/strong signal, or you'll soon find yourself locked at a high SNRM and not being able to get rid of it. If you have a high attenuation you may not be able to hold onto a 'Normal' setting and a 6dB SNRM, in which case a 'Stable' setting with 9dB is the best you can hope for.
Hope this helps others and their deliberations with their ISP.
[attachment deleted by admin]