After much wailing, reflashing and gnashing of teeth (http://www.geekzone.co.nz/forums.asp?forumid=65&topicid=35664&page_no=3) I've finally obtained the following line stats:
Line Rate: 15735 Kbps 945 Kbps
Attainable Line Rate: 20076 Kbps 945 Kbps
Noise Margin: 12.3 dB 16.2 dB
Line Attenuation: 19.0 dB 8.9 dB
Achieved with a dynalink RTA1320 in half-bridged mode connected through a WRT54GS running tomato 1.25.
(Thanks to: http://www.ben.geek.nz/adsl-routing-solution-in-detail/)
...and yes, the real world on-the-browser experience is much improved.
At last!! :)