.67 M1 could be achieved at FL 410 or 370 for that matter in the P.180, albeit not at MGTOW. This required a higher power setting than what I normally used, and I found that for the best combination of speed and range, a power setting yielding about 370 ktas worked best.
The CG range in the airplane isn't that great. Single pilot, empty with full fuel can put it aft of the envelope, and nearly uncomfortably so, and we found that in normal operations we were nearly at the edge of the envelope much of the time. Often creative use of the loading schedule was necessary to balance the aircraft.
Due to the laminar-type airfoils used, the airplane also displayed a very unusual characteristic which is more reminiscent of early experimental canard-style aircraft; any disturbance to the airfoil causes significant loss of lift. This was most apparent when hand flying and passing in and out of fair-weather cumulus clouds. Upon entering the cloud the control column required movement aft of two inches or more to sustain level flight, else a pitching-down moment would occur. When exiting the cloud, the opposite movement was required. This see-saw action was very apparent when hand-flying in and out of clouds, but transparent when on the autopilot.