The thing is, Annex-M has become an anachronism in most locations. It was only ever worth it on shorter lines -- the very lines that now get far better performance with FTTC/VDSL.
The problem is that the upper half of the frequency band for the Annex-M uplink is used for downlink on everyone else's lines. So crosstalk is a huge problem on longer lines. While bins 7-31 and 64-96 perform strongly, bins 32-63 can support surprisingly few bits, due to crosstalk.
I'm on a 3 km line, where FTTC won't do much for me. I migrated from Plusnet 20CN (Pro) to Pulse8 a couple of months back and have been struggling with this very issue (wholesale TalkTalk Business). To its credit, Pulse8 actually volunteered to downgrade me to Annex-A, because I wasn't getting value for money. I've not taken them up on the offer (yet), but I estimate I'll lose ~300 Kbps up, but gain ~1500 Kbps down.