Just by way of an update, my IP profile seems to have been revised to 2Mbps (from 2.5Mbps). Seems a bit more stable now as it wasn't really coping at that speed with or without the extensions. Sync varies from ~3.9Mbps to 3.1Mbps and seems to stay up without much problem. Less so at the higher rate (SNR falls below 6dB), but sitting at 3.1Mbps just now with 7.5dB SNR.
So throughput tests running at close to the profile of 2Mbps, so definitely an improvement, but still somewhat lower throughput than my neighbour. It doesn't bare thinking about trying to eliminate a potential source for electrical interference in my house. I have literally dozens of PSUs (switching and coiled), fridges, freezers, etc. etc. I think it is just as likely that my particular copper pair is impaired in some way. If I feel the need, I could look at shortening the RJ11 cable, removing the other PSU's in the room the router is housed and see if that helps.
This is all running on the DG834G v5 I bought. Will swap back to the DrayTek Vigor 2800 to see if it's any different.
Thanks for the all the input on this thread!
