I've been doing some digging around and found a tip that the following Telnet command:

adsl codinggain 8

gave a 20% improvement in the sync speed compared to the default value of 0 (Auto.) I'm still not getting the same results on the broadband speed tests as I got with the D-Link but the difference is narrowing.

I wonder what other undocumented commands there are that can improve speed?

There is a FAQ on this exact subject -

http://www.draytek.co.uk/support/kb_vigor_inp.html