I noticed a decent improvement when changing from the HG612's bundled ADSL cable to an ADSLNation/Tandy cable.
The bundled cable was really very long, so initially I just cut it to size and connected it to the data extension terminals inside the faceplate. This cut attenuation from 22 to 21.5. Then, after an OR engineer (rightly) told me off for using stranded cable in an IDC punch-down terminal, I switched to the ADSLNation cable. That took me down to the present 20.5 (although as the weather warms, it's crept up to 20.6).
Unfortunately, the ADSLNation cable's still a bit too long. I'm almost tempted to cut that to length too and use the data extension terminals again. But, law of diminishing returns and all that.