Pretty sure my main line is the old ADSL Nation (marked as ADSL 1.0), it performs no different to any of the dangling microfilters supposedly designed for VDSL in my experience.
Interesting but perhaps not surprising? The ADSLNation filter was supposed to be one of the best in its day, wasn't it? And I have the (uninformed) impression that dangling filters range from good to poor, at least in theoretical terms. I've no idea whether changing filters makes much difference in practice, and if it does, which design works best will probably depend on the line.
This post from 2012 compares the ADSLNation with presumably the BT Mk3 SSFP and found no difference in sync speed.
I don't even know how or why the filter affects the DSL signal. I presume the input stage of the modem ignores anything at speech frequencies, so we're only concerned with how it might muddle or attenuate the RF. A quick search of
Google Scholar turns up some references to active filters (ass opposed to active components), which I guess could be the future – unless we just give up on wired telephony altogether.
I had a quick look inside a Mk3 and it looks quite similar to the Mk4, with four large inductors.
Bcat suggested they may be identical.