My back-of-the-envelope says M=0.74, assuming ISA.
Seems to be too far below M=0.86 to really matter....
I came up with about the same and I agree.
There is a discussion on the Pilots Forum regarding this as well. Groundspeed data computed by the FMC, when adjusted for wind is within 1-2 knots airspeed. Obviously, there weren't any problems with the Static System.
It appears the reason for this whole thread is that a man named Frank Legge, a supposed chemist from Australia has been trying to discredit pilots who have questions regarding 9/11. His paper was posted a few pages back in this thread. You can read more from the pilots here with respect to this topic.
Frank Legge Begging For Peer Reviewers For Pentagon Paper - Pilots For 9/11 Truth Forum
Here are their list of members.
Pilots For Truth List Of Members
However, the same man (Frank Legge) who is trying to discredit pilots, without being a pilot himself, claims the WTC was destroyed by Controlled Demolition. He seems to trust the data provided by the NTSB, but doesn't trust anything from the NIST. Go figure.