Never seen a temperature differential on a A/C unit nor a heat pump, maybe different terminology.
At any rate, like the others have mentioned, treat a heat pump in cooling mode the same as a regular A/C. Hard and fast rules don't really apply for either, temperature differentials are just one factor of evaluating a system. A properly functioning system might have as little as a 11 degree differential between the supply and return air temperature or as much as a 20 degree split. It depends on the unit and the installation. Higher efficiency units will have less differential than older less efficient ones as a general rule. Differentials will also vary according to the weather and indoor ambient conditions, including humidity load.