Openreach specifically state they don't want you altering the SNR. In any case, I tried it with an unlocked HG612 and it made a complete mess. The reason possibly being that while on ADSLx the upstream band is below the downstream, on FTTC they are split into sections and the sections alternate.
Discovery Phase (Initial) Band Plan
US: (7,32) (871,1205) (1972,2782)
DS: (33,859) (1216,1961) (2793,3970)
Medley Phase (Final) Band Plan
US: (7,32) (871,1205) (1972,2780)
DS: (33,859) (1216,1961) (2793,3970)
You can't affect the interleaving except by having a clean line or reducing your sync speed, which will reduce errors so help with the interleaving. Someone has managed to do that, I think it was with an HG612. What is yours?
Given a stable line with low errors interleaving does turn off automatically after a few weeks. I managed to get it turned on last night

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My broadband basic info/help site -
www.robertos.me.uk. Domains,site and mail hosting -
Tsohost.
Connection -
Plusnet UnLim Fibre (FTTC). Sync ~ 52.8 (interleaved)/15.0Mbps @ 600m. - IPv4
BQM IPv6
BQM