The P180 Avanti has been tested up to MMO of 0.7, this means that it has been pushed to 0.77. At this Mach No (0.77) it has also been tested for flutter, meaning bonkers has been fired (bonkers are pyro devices which ‘explode’, giving a knock on the surfaces. All modes were tested : wing symmetrical, anti-symmetrical, same for canards, same for fin, same for elevators) ; never the mach-tuck phenomenon has been encountered. Buffet onset envelope has been investigated up to 1.4 g turns, with no negative results. Static stability has never relaxed up to some mach beyond MMO
Max TAS of 402 KTAS (on Avanti II) can be reached easily , but at 26000 ft, surely not at FL 370.
I was there in 1989