Hi JK
Max is a rate-adaptive service, which means that it will run as fast as your line allows. That's the theory. In practice BT uses a system it calls profiles to reduce load on its computer systems. This system groups sync speeds (which step in 32k increments) into bands, for which a profile (or throttle) is allocated. The profile is always below the minimum level for the group and well below the maximum. For this reason, if you live on the cusp, so to speak, a 32k sync speed change can change your throughout by 500k.
For your current sync speed, you would get a profile of 4000kbps.
It is normal for the noise margin to change during the day. It will be at its highest in the mornings and lowest at night. This effect is due to increased electrical activity at night and the increased propagation of radio waves - ADSL operates in the MW band.
TBH, for a 39db attenuation, I'd expect you to be seeing speeds around the 6000kbps mark or a bit higher. It may be that there are things that you can do to improve your speed. Do you know if you have an NTE5 master socket, the kind where the bottom half of the faceplate can be removed? Do you have multiple hard-wired extensions? What router/modem are you using? What else is connected to the line, eg phones, Sky box, fax machine etc.