BT have actually made a real mess of the DSL Max upgrades - none of the trial lines we have set up have received anything better than 4mb, despite being sync'd at 8mb. It seems that their tool that judges the maximum stable rate of the line (based on the speeds obtained when your connection re-trains) was not sensitive enough, so if your line didn't retrain at 8mb enough times during the 10-day period they test it over, it wouldn't log enough variations in speed to increase the maximum stable rate.
*deep breath*
The have (allegedly) fixed it this week, so if you just reboot your modem/router a couple of times a day over the next few days, it should pick up on the changes in speed.