Having a look at my parents connection and the speeds are pretty poor(DSL-G604T). I don't think it's ever connected at more than ~5,5
First thought was the modem firmware might be limiting it to adsl speeds but plugging in mine(TG585 V8) proved no better in sync speed and swapping out the existing filters made little difference. The low speeds have been constant(4-5mbit) since moving from Iconz(who at the time didn't offer adsl2 and was FS/FS) to Telecom a number of years back.
The second modem I tried works fine at another property/connection at adsl2 speeds so my assumption is it may be back-end related(still on the same profile as it was with Iconz or something) if that makes sense.
Any suggestions as to what might be the cause? I don't really have time for phone-tag but can PM the associated details as required.