On the one test unit, I will now try 250/0750, hardware reset then software reset. Will see what happens! One thing I am wondering is if we are triggering an internal chip overheat of some kind, that throttles it back but doesn't report a bunch of failures. So we try to push them harder, and they actually go slower. But I'm just guessing at this point.
Its your rig, but I'd try 275/0800 first. Simply enter the voltage, save and apply then do a power cycle.
On pushing them harder if they are going to throttle back without showing any outward sign ..... that is the question.