I treated myself to a DGN2000 for Christmas, and flashed it with the DGTeam firmware. I set the "SNR percentage" slider to the minimum value, which brought my target noise margin down from 15 dB to about 9 dB. I then left it running for 24 hours.
My impression was that the performance was not as good as the DG834GT - there were a couple of unexplained re-syncs, and the noise margin graph as shown in Routerstats appeared to be more jittery. So I put back the DG834GT (with identical settings) to get a straight comparison. The DG834GT connected at a higher speed with the same noise margin, and the noise margin graph was much less jittery - see the screenshots below (the left half is the DGN2000, the right half is the DG834GT).
So my initial conclusion is that the DG834GT has better performance than the DGN2000 on my line, at this time. Of course it's not an entirely fair comparison because I'm using unofficial firmware. I will have a go with the official firmware at some time, but at the moment the DGTeam firmware offers features which I want to retain, so I'll stick with the DG834GT.