I am currently on O2 ADSL2+. Up to a few months ago I got a very stable 10-11Mbs connection with a 42dB line attenuation. The local cabinet was upgraded to fibre in December and I soon noticed my ADSL speed had reduced and it now hovers between 9Mbs on a good day and as low as 6.5. The stats show a high SNR but the FEC errors are very high, millions of them a day, so it is unstable at a 6dB margin.
I suspect crosstalk between lines carrying fibre customers and those on ADSL. I notice on the BT site it indicates an estimated speed for my line of 5-7Mbs, much lower than it did when I checked several years ago. Alternatively of course they may have disturbed the wiring while blowing the cable through. It estimates a fibre speed of around 30Mbs, the cabinet is around 1km away - tempted but I don't really need fibre as I do little downloading.
Have others noted speed deterioration after fibre has gone live in their areas?