This Openreach PDF
, dating back a while, has a *great* primer on VDSL2 - including aspects that answer almost all of your questions.
In particular, it mentions band plan 997 for symmetric services, up to 12MHz. The basic variants of VDSL2 start by using 8MHz; band plan 997 dictates how the frequencies are split between upstream & downstream.
The PDF then mentions the fact that BT are using a truncated band plan 997 - which cuts the tops frequency to 7.05MHz - because this is the maximum frequency allowed by the ANFP in Openreach's cabinets (defined by Ofcom). The frequency lost would have been used by upstream data, so doesn't affect the overall downstream rate.
It then mentions the fact that the VDSL2 kit in the cabinets must employ extra power-control. This is factored-in, based on distance from cabinet to exchange, and is done to ensure that where the VDSL2 frequencies overlap the ADLS2+ frequencies, they have the same apparent strength to subscribers further out on that cabinet. This stops VDSL2 from swamping the ADSL2+ subscribers.
The PDF then mentions a little about the profiles - which really define the frequency use, and the power levels. Some profiles are intended for use from exchanges (so are higher), while some are intended for use from cabinets (and are lower). 8c is intended to be used from cabinets, so is lower power, but also brings in BT's frequency cut-off of 7.05MHz.
Finally, crosstalk... Yes, it will happen, but profile 8c, and the extra power-control feature, are meant to be measures to reduce the impact.
You can find a number of graphs showing how fast FTTC can be - with some giving estimates of 100Mbps. However, a graph that shows the effect of distance on speed in the presence of crosstalk shows the extreme speeds are only really available within 100M. There is one such graph in this document by Ericsson
(page 8), which shows more realistic speeds for profile 8d (8c will probably be similar) with crosstalk from 20 lines.
My feeling is that BT's estimates are making predictions of speed with crosstalk present (and match that Ericsson graph reasonably well), which is why the first people are getting much higher speeds... for now.
The graph suggests a path limit of 1500M - and limited by the upstream, not the downstream!