The command is there and it works on adsl, but on VDSL the command is over-ridden by the BT DLM.
The BToR DLM doesnt use SNRm as a configurable attribute (it uses banding and interleaving) and a default Target SNRm of 6dB is set on all lines, so although you can attempt to use it, it just gets over-ridden by the DSLAM. :/
You can however use adsl configure --maxDataRate to adjust the banding downwards which will give you a higher SNRm.