From experience, to accelerate into "Hollow yellow bar" whether or not in turbulent air,will induce airframe buffet...This of course at high level...If the system detects a compromise of the .3g protection "BUFFET ALERT" will be presented on the CDU...As for high speed handling characteristics I'm no test pilot but MACH TRIM operates above .617MACH...with speed limited to .74MACH with the MACH TRIM sytem inop...