regarding your RCO (Max Attainable Rate) that was a LOW estimate..
I dunno if you been reading my other post about finding out (probably) why DLM falls short of the mark... in a nutshell its so SKY save a fortune in energy costs... the output power is lowered resulting in lower sync rates but power consumption is much much less in comparison, for e,g, a typical ADSL2+ line would be 18dBm using 64mW - but a 10dBm line that can sync at 75% the speed of the 18dBm line uses SIX TIMES less energy at 10mW, this is because for every 3dBm increase the output power DOUBLES (0dBm = 1mW , 3dBm = 2mW, 6dBm = 4mW, 9dBm = 8mW, 12dBm = 16mW , 15dBm = 32mW , 18dBm = 64mW..so on and so forth), its no wonder SKY have sneakily deployed this system at cost of customers sync rates...
all makes sense now..
anyway, heres why DMT Tool or whatever reports a low RCO or Max Attainable Rate, I read it on KItz while studying to back up my claims, because the output power is lower than it should be then the true RCA=O can not be calculated and thats why yours says 15Mb....and possibly why SKY reps see the same..
~ RCO - Relative Capacity Occupation
The existing sync rate expressed as a percentage against the Maximum Attainable Rate.
For example if you were sync'd at 8128 kbps because of restriction at the dslam or on your account, but the router estimates that your line is capable of 16Mbps, then the RCO would be 50% as you would only be using 50% capacity.
Something to be aware of though, is if your line is not running at full power because say of adsl1 restrictions, then it may not be able to reflect the RCO for adsl2+ accurately.
Read more: http://www.kitz.co.uk/adsl/linestats_explanation.htm#ixzz1p2ahSzH4