I"m not sure about the max revs of DMU engines, but I can certainly attest to fact that they spend far, far longer at full throttle than an HST on the HML!
An HST only needs notch 5 (normally) to reach line speed, an intermediate notch is usually enough to hold line speed, once you reach it (evrn on a rising gradient). Whereas a DMU is pretty much on full power from, for example, Blair Atholl to Drumochter summit.
I've heard of a Scotrail HST running on one power car maintaining line speed on this part of the HML, which, if true, is truly impressive