The chipset in that router is designed for longer lines, it may be as Steve suggested, the BT2700HGV gave me a 5Mbps line in my old home when I could normally only really manage about 3Mbps, that router was an exception to the rule though. This routers chipset gives very good performance, better on longer lines than say Trendchip ones which work well for short lines.
My D6300 is giving me the same stats as my DGND4000 and old 3700v2 so it might well be down to that old 2 Wire just being exceptional, but going forward you may have to settle for this, unless you can use it as a modem and the 3700 as the wireless router, I cant remember enough about them to know if that's possible.
Did you update the firmware, Den, and do a hard reset afterwards? Also have you had your line re-profiled? As the upload is great but the download seems low for ADSL2+ Also I know you said its a v2 but there are some Original DGND3700's still out there, it does says v2 on the back near the reset button where its says DGND3700