In the article: I heard the click of Walter’s mike button in the rear seat. In his most innocent voice, Walter startled the controller by asking for a ground speed check from 81,000 feet, clearly above controlled airspace. In a cool, professional voice, the controller replied, ‘ Aspen 20, I show you at 1,982 knots on the ground.
1,982 knots = 2280.85 still a bit faster then official numbers.
Stall speed at altitude is pretty high. Un-starts aren’t pretty.
JP-58 turbo ramjet engine video with charts:
https://www.youtube.com/watch?v=F3ao5SCedIk
3:13 into video see diagram for various power settings and altitude curves, with mach figure on bottom scale. Maximum efficiency at a bit above M 3.2 as indicted by fuel burn factor on left margin.