My experiences if they of interest to anyone - when I was switched to ADSL 2+ overall there was no noticeable speed improvement with most speed tests both before and after the change showing about 4.8 Mbps on my screen (the router typically reported 44db attenuation with an snr of anything between 4 and 8db). Subjectively the line seemed more stable though with fewer disconnections.
At the beginning of this month I switched to FTTC and download speeds now average around 27 Mbps with a low of 21 and high of 32. Upload speeds have improved from 375 kbps to around 1675. I have not detected any disconnections at all.
My ISP is Plusnet.