I think you hit the nail on the head....
That graph shows that at 400m you should get around 20 Mb/sec. Well my line has D side link of 400m approx and I get 35 Mb/sec against a predicted 22 Mb/sec and whilst I understand the discussion about crosstalk my speeds have been stable on a cabinet that has been available for 12 months and if the number of times an OR van is parked beside it they appear to be selling lots of connections.
My D-side is 566m long, and I have held perfect 40000 meg sync down, and 10 meg up sync it's installation in November.
Attenuation d/s 18.2db snr 7db sync rate 40000Kbps
attenuation u/s 12.2db snr 17.3 sync rate 10000Kbps
I have seen full sync on 890m of 0.5 copper, (new-ish build buried estate) but then have seen only 16meg on 700m, this being almost entirely aluminium from cab to pole. Before anyone shouts, we swapped the entire length of this slow line, and it still did not improve, so this wasn't due to a 'fault'.
Hasn't everyone one here who has been given an estimate found it to be lower than the eventual speed ?
I have seen 'unconfigured' FTTC DSLAM ports show 80meg sync downstream, though how far this would travel, I don't know, (possibly 8a rather than 8c.) There are currently around 40 users off my DSLAM.
So, yes, I believe it can go faster/further than estimates.