Some photos of the internals of your phone sockets, showing the wiring, might be useful.

given you get the same at the extension, (via a/b) as you do at the test socket, there is little you can do by improving internal wiring.
[but if you wanted to use the router at the master faceplate, then connect extensions via 2/5]

so, really its try a different router,

there is little you can do by improving internal wiring

yes - this was my view too. Disappointing as I was hoping to eke out a bit more sync by completely removing any extension cabling.

I've tried a 2Wire, as suggested, recently - holds rock steady at 6 db and syncs in 1600 to 1700 kbps range. Currently using a Billion 7800N with a bit of SNRM tweak: gives me between 1750 and 1850 kbps and again the sync is rock solid down to 3.5 db or so. I think that's probably as good as it's going to get unfortunately unless anybody has any other ideas.