Aces II max speed was always listed as 600 KIAS (at least in the F-15). There was an F-15E crew that got out at 780 KIAS below 10,000 feet after getting disoriented at night. WSO was killed, front seater recovered after a long rehab and went back to flying.

So, wouldn't settling this "bet" using KIAS instead of Mach be a better method. (although I guess the ejection above actually disproves the .99 limit - 'cuz 780 below 10K is mo' definitely above .99).

It would seem like ejecting supersonic at higher altitude would have less potential for injury than the same Mach number at lower altitude. Indicated airspeed will be lower the higher you are (i.e. less of those air molecules to batter your body in the more rarified air).
Am I out to lunch?