For anyone else interested, I did upgrade from my 25/5 to 100/40 even though I knew that I would likely only get maximum of 50/16 sync. But I was happy to pay for the speed increase such as it was, particularly upstream.
I am pleased to advise that the speeds I got were exactly the same as the attainable rates reported by the modem. So, around 51/16.5. From the tones graph, the use of ADSL2+ spectrum is reduced during this coexistence period and therefore I am confident that I will get an increase in speed as more people in the street drop off ADSL2 and ultimately the coexistence period concludes.
VDSL2 stability has been fine.