Maybe ... On my old Speedtouch with standard ADSL it report 19.8 dBm occasionally dropping by a 0.1 or 0.2. When I moved to ADSL2+ it tended to report 0dBm but would often give reading up towards 20dBm. As most people tend to look at router stats when the link is quiescent it could be that the 0dBm reported by the DSLAM at that moment in time.

I have the same attenuation as you and am also on WBC ADSL2+.

When I had a 6dB NM, as you have, I was getting nearly 17 Meg sync speed.

On stable lines WBC DLM can reduce NM to 3 dB, on which I am now getting over 20 Meg.

So there is hope for you yet, while you wait out the training period, but you will never get the 24 Meg you assumed.