If you lose sync, then even if you do tweak, you might get better speeds for a while, but it will eventually either
(a) settle somewhere near the speed you're already getting anyway, or
(b) constantly lose sync / reconnect, which would probably end up with throughput being slower overall, as the noise margin will be less, therefore more scope for line errors, slowing your throughput down.
Keep an eye on your sync speed and SNR at periodic intervals today. If your router resyncs by itself to a lower value during the course of today, there's no real point trying to pursue the DMT route, as one of the two scenarios above will happen. Tweaking the SNR margin down below O2's standard 6.5dB is only of any use on ultra-stable lines.