I don't think BT will increase the top package speed of 80/20 through this. As mentioned, they saw the benefit of vectoring as a way of making speeds more consistent rather than faster - and vectoring probably has more potential for speed gain than a variable SNRM. I should reiterate that it doesn't have to be a 3dB target. Changes of 1dB are plausible too, just resulting in less of a gain.
I'll add a note that I was first inspired to calculate the speed equivalent of 3dB when Eircom turned on vectoring, and it still looked like BT would be too. Eircom don't use DLM, and turned on both G.INP and Vectoring at the same time, but left their standard SNRM target as 9dB.
It'll be interesting to see what mechanism they use to deploy and vary the settings.