No, it is determined automatically by the exchange. It is called DLM (Dynamic Line Management) and it outside Plusnets control.
When the DLM training period is started it uses a min SNR of 6dB and syncs at whatever speed that will achieve. If the DLM sees lots of disconnections or noise on the line it raises the min SNR by 3dB and resyncs. If it still sees noise / disconnections it raises it again by 3dB. It keeps doing this until it gets a stable connection or reaches 15dB.
The theory is that it should also work the other way where if it sees a good connection it should reduce the min SNR and resync. However in practice this rarely happens and people get stuck on a slower connection. The IP profile is related to the sync speed and this determines how fast you can download/upload data. As the sync rate lowers the IP profile (Data rate) lowers. Getting the Ip profile up again when the sync rates goes up is also suppose to happen automatically but often gets stuck.
Plusnet can in some cases get the retraining to occur again but BT Wholesale can be very difficult and say no.
What are your router line stats? see
http://www.kitz.co.uk/adsl/frogstats.php
