Err.. Yes you are correct. The upper speed is limited to Mcrit or the FAA imposed Mmo (Mach max ops.) So maybe not related to engine power, as that speed could also be achieved in a dive.
Quote from Wiki...
As an airplane moves through the air faster, the airflow over parts of the wing will reach speeds that approach Mach 1.0. At such speeds,
shock waves form in the air passing over the wings, drastically increasing the drag due to
drag divergence, causing Mach buffet, or drastically changing the
center of pressure, resulting in a nose-down
moment called "
mach tuck". The aircraft Mach number at which these effects appear is known as its
critical Mach number, or Mach CRIT. The true airspeed corresponding to the critical Mach number generally decreases with altitude.
Datei:CoffinCorner.png ? Wikipedia