After moving house I have been lucky enough to sync at the full 8128kbps (apart from a bad sync lastnight), it sync'd at that for 7days. It took a couple days for BT to update my profile to 7150k on there speed check page, plusnets then went to 7000k. Now I know 150k isn't much but do plusnet not use the top speed ? The online pages suggest that the profile should go to 7150k.
On a different note not aimed at Plusnet its annoying when you hit the top of the sync rates the difference between sync and profile increase to a 1mb difference. I can't see how any ISP can advertise "up to 8meg" when under ipstream the most you will ever see is 7.15meg.