I have a similar customer with a line capable of 5 mbit/s but is synching at 2 mbit/s with a 12dB noise margin. BT had reduced the sync speed due to 'errors' and despite me doing the "replace the entire CPE kit" thing they usually request prior to doing anything, the sync profile was reset and made very little difference. Customer chose BT over us and was reluctant to jump through the BT hoops, so they sit at 2mbit/s.