We live in a village that is getting BT Infinity as part of BDUK. The BT Openreach engineer said that BT Infinity which uses VDSL2 performance drops off dramatically (like a cliff were his words) when the copper line length exceeds around 300Mts. Any Aluminium in the line also degrades speed quickly. Also, as more Infinity customer are in your cabinet / line then cross talk increases and speed drops. BT are now getting requests to go back to ADSL2 after an Infinity 'upgrade'.
I found a chart from apparently Ofcom which shows how VDSL2, VDSL and the ADSLx compare by length of line - http://nbnmyths.wordpress.com/why-not-fttn/.
My question is why does BT persist with VDSL2 when for longer real world line lengths VDSL gives better performance?