@cbdeakin I would not think that a small drop in Sync rate [from 50 to 49.13] would make any difference to your errors. Sync rate will vary like that in both directions depending on interference on the line at the particular time the ‘handshake’ occurs. Very often (especially on longer lines), the sync rate will be lower at night after the sun has gone down.
If a connection errors heavily, it will do that on either a higher or lower SNR margin. The SNR is basically how much scope has the DSL signal got to hang on. In the olden days target margins were between 6gb and 15db, whereas now 3 is usually the target, or banding is applied in extreme cases. Imagine the SNR like this, 15db is a thick brick wall, 6db is a thin wood wall. If you hit the wall with a sledgehammer, which one is likely to break easier? The thin wood wall, hence why the lower the margin the less room the connection has to hold on. Both walls would experience the same weather (errors on a DSL circuit).
Giving customers the ability to alter SNR margins would not be a good thing in my opinion: 1) because most don’t care and 2) if people start playing with it then it will mess up Dynamic Line Management. Having said that, in a very limited number of circumstances, if service providers like Cuckoo were able to do it, in situations where DLM does not work properly, they could apply different settings to counteract unfavourable DSL conditions. I know I mention my Fritzbox far too much, but essentially I can do what you are stating in your post. I’m having to up the Impulse noise protection because DLM won’t put my line on either vectoring or retransmission.
Having said that, on balance, I would highly caution against what you are suggesting.