The entry for airframe type is in the entry for primary aerodynamics, section 1101. Basically, airframe numbers are designed by Microsoft for a typical aircraft type, so the airframe number for the plane you are flying is probably already approximately right.
What you need to do is amend two other settings in the .air file usiing AirED: one is line 316 (I think - haven't got access to my PC at the moment) called Mmo, which is the maximum Mach speed allowable. You can set this anywhere up to 3.199, which is the maximum speed allowable in FS without turning off the stress crash option. Then, in section 1101 (above), change the maximum indicated airspeed in knots figure to a higher one. Problem should now be solved.
Whatever you do, don't try to make a Concorde fly faster, or use the dedicated Concorde .air file for another plane, as the Concorde .air file has a dedicated airframe number (5943, from memory) which has been hard-coded by Microsoft to bring the overspeed warning up at just before M2.1, regardless of what you do.
Good luck!
