I have used the test socket for a few days and noticed no difference at all, really. This must mean the extension is fine? My SNR doesn't tend to vary by more than 1dB (1.5 at the most) in any given day in the test socket or faceplate.
My neighbour claimed to have had ~50mb download since 2013, after initially connecting at 65mb. He tells me his connection is very stable as well.
It's obvious my line is close to its capacity, but that capacity seems to be sub-par. I just wondered why the discrepancy with attenuation was only on the upstream, and an upload of 5mb on my attenuation definitely seems suspect: used to get much higher on a longer line - the length of copper to this line has been confirmed by an engineer as ~660m.
If new wiring wouldn't increase the upload, then I'm satisfied that there's nothing reasonable I can do here: not that my stats are too disappointing. My neighbour getting 9mb does irritate me, though
I will ask him to get some specific stats.