550GS only apply to fighters below FL400. Even then, it was temporary for airliners until overspeed violations became hard coded. You won’t get to and maintain that in any airliner or GA as it stands right now due to violations being recorded past M.87.
Correct, Mach number is your speed relative to the speed of sound. This is because as you approach the speed of sound (ratio wise) the drag increases very fast. Groundspeed has nothing to do with Mach at all.
Each aircraft has red dots on the speed tape where the speed warnings stop. It depends on altitude and a number of other factors and could be different for each aircraft. Fighters over FL400 can go faster if I remember correctly.
While speeding on expert has been reduced, we still encounter people who ignore the warnings and ATC has to respond accordingly.
We will not know about the other aspects of global until it comes out. The restrictions are in place to help ATC out due to the unrealistic volume.
Yes but it is no longer ground speed which is measured as a restriction but a hard coded overspeed warning. It remains M.87 above FL400 same as at FL280, below this it reverts to 350 IAS down to 10,000ft and below this 250 IAS.