I know there is a requirement to report a change of speed to ATC when the change is bigger than .... kts (something; Alzheimer knows
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Same goes when an aircraft is locked om Mach number and he needs to change the speed with M0.01 or greater.
However if the speed is published within the Flightplan there is no need to report it as, in a way, it is already reported.
However in any case if the performance of an aircraft is altered beyound the expectations for that aircraft, although published in a flight plan, a head's up to ATC is more then appreciated.