Regrettably I do not think this can be changed by the customer or BT Wholesale. If the noise margin becomes really silly (e.g. 10dB on infinity) you can try to get it reset but it requires a BT Openreach engineer with a job number for your line and persistence on his part.
I think the target should be around 5.9-6.0dB and at 6.5dB you are only slightly above this so close to maximum bandwidth. I'm guessing 6.5dB is a current rather than average figure, so it may be at some points during the day the noise on the line has increased and so your current noise margin. Also bear in mind that depending on the chipset in your modem the noise figure may be more of an estimate than an actual measurement. However the high noise margin may be due to the BRAS profile so you have my sympathy as in my opinion 'the system' limits the sync rate rather too aggressively.
