>> successfully increased my sync speed by more than 33% and it has remained steady for 4 days continually.
Good stuff.
>> but see some HEC and some OCD errors. Should I be concerned by these?
Your line should be able to cope with a certain amount.. a problem can occur if it starts racking up 100's over a short period of time.
>> I still have a S/N =9 and I wonder if I could shave that even more to increase the speed speed.
There is only so low that you can go with DMTtool since it works on a %.
>> Is there anything I can do at this stage to encourage my ISP to re-rate my line to this new speed?
Once its been stable for a while at the new SNR - you can but try.. and could always contact them saying that its been fine by you manually adjusting the target SNR on your router and that its been fine for x period of time so could they get BTw to adjust it. Enta ISPs are normally ok about this. (unlike some others).
Hopefully though if the line is stable the DLM should do it itself, but how long it can take is up for debate, and it also seems to depend on how many errors your line is reporting.