Hope this contains all the info you need

Default

Tweaked

As you can see, the reduced SNR actually has a slightly slower sync than the default

interesting. Does it do bit loading plots ? Seems it finds the same speed at a lower margin, which implies selection of (or preference for) less optimal tones.

Maybe someone else can answer that. Not entirely sure what you mean

On the bright side, BT have been installing fibre cabinets over the past couple of weeks

some routers show how many bits of data are allocated to each frequency band in the DSL spectrum. That might illustrate what's going on. You see a bar chart that declines in height as the frequency increases, usually - with more bits per band at the low end where SNR is best.

Ahh. I've not seen anything like that in there