That's a really disheartening experience.
I've been more fortunate so far (switched 6 days ago) -- there has been a fairly consistent 2Mbit improvement in sync and download.
sync has gone up from around 6.8 to around 8.9 Mbits (it hit 11 Mbits at one stage); and download has increased from approx 5.1 to approx 7.2 Mbits.
Downsides are (as you have also experienced): inconsistent target SNRM; and no interleaving. But as this is still within the initial period, I'm reserving judgement...
I actually had to use DMT Tool to increase
the SNRM massively at one point, because it was set so low at the DSLAM (0.5 dB!) that the modem would not sync at all.
As for the interleaving, my line is relatively severely affected by commercial AM radio, so even with a high SNRM set (12 dB) the incidence of CRC errors is not good (hundreds per hour). With interleaving on, the CRC rate was in single figures per hour, with FEC dealing with the rest.
I imagine that it's just a question of getting MAAF to switch interleaving back on...
What sort of speed do they think you might get on fibre? The Plusnet site suggests we might get 24 Mbits, which is very tempting, but also a huge increase in price from what we're paying now.