To be fair I don't have complete confidence in what this TalkTalk engineer is saying.
He's convinced he can make the connection more stable by changing the DLM profile but he can't as ISPs have no control over FTTC-DLM, except the line profiles which only change when DLM makes changes to the line.
I am fairly certain TalkTalk put FTTC lines on the stable profile anyway.
If their tests keep passing I'm basically in limbo because as far as I know, Openreach don't do changes without hard evidence