Its not reliant on attenuation nor the depth or delay of interleaving.
Its actually to do with something called S=1/2 mode.
Standard ADSL1 architecture has a maximum download speed of 8.128 Mbps due to framing overheads.. or more specifically a limitation of the maximum
Reed Solomon (RS) codeword size.
s=1/2 mode allows 2 RS codewords to be interleaved and mapped to one data frame which in turn greatly increases the maximum acheivable downstream speed up to the limitation of the maximum number of bits that can be carried across the available
DMT subchannels. (num of subchannels x 15 bits per subchannel x 4kbps).
If all DMT subchannels were in use - which theyre not - and if the SNR in each subchannel was good enough to carry 15bits per tone... now we are talking near 16 Mbps.
DMT (in the UK) generally uses downstream tones 33-254 so we now have a maximum download speed of somewhere approaching 13Mbps.
BTW - Are these figures ringing any bells with adsl2 anyone? - ignore adsl2+ since that works by doubling the no of available subchannel frequencies.
So how does this fit in to the question?
If you have a router that supports s=1/2 mode...
and the dslam you are on is also configured to supposrt s=1/2 mode (not all are),
and if your line is good enough to be able to
load sufficient bits per subchannel.... then an interleaved line can over-ride the maximum 'normal' limitation of 7616 for an interleaved line. You wont ever get more than 8128 though, since that will be a hardcoded maximum configuration of BTs dslams.