For background on why this is required, see My Vigor router does not seem to be invoking QoS. I think the problem is that online status reports my connection downstream speed as 1.5Mbps – but my ISP plan has much less bandwidth.

Yes from Vigor 2800 firmware v2.6.3; but at the moment it is only available as a telnet command.

> adsl rxpct ?
rx percentage : 33
% adsl rxpct [auto:percent]
% auto : auto detect, percent : 10-100

Note that a rx percentage of 33 tells the router that the actual receive (download) speed is 33% of the synchronised speed (reported by Telstra DSLAMs as 1.5Mbps) – i.e. 512Kbps.

There is also a tx percentage value, but we recommend keeping this at auto because Telstra DSLAMs do report the upstream bandwidth correctly.

> adsl txpct ?
tx percentage : auto
% adsl txpct [auto:percent]
% auto : auto detect, percent : 10-100.