I don’t see why this would matter at all. Maybe the heat exchanger would need to be sized differently for a different flow rate, but in general a lower entering water temperature on the hot side seems preferable.
That's why designers are often specifying lower delta-Ts for low-temperature emitters: to allow the flow temperatures to remain as low as possible [for efficiency] at a given average water temperature [for effective heating].
Maybe it never comes up in practice because heat pump systems inevitably need some kind of buffer tank. If you're designing a system from scratch then you design for lower delta-T in the emitters to keep the max water temperature down. And if you're using existing emitters then you just live with the inefficiency due to higher max water temp, but still keep the flow from the heat pump to buffer tank high regardless (to keep the max water temp from being even higher).
So you can't just take a decades old system with oil/gas using finned radiators, just replace the boiler, and have it supply enough heat on the coldest day ("design day"). Rather you'd at least need to add some emitters, greatly increasing the scope of the project for a professional installer.
What I haven't been able to find an answer to is that everybody says hydronic heat pumps need low delta T of 5-10 degree F (implying high flow rate for given heat transfer). But I would think the real constraint would be just on their leaving water temperature, and a heat pump (load side) that took in 100F and put out 120F (at say 5GPM) would be happier and more efficient than one that took in 110F and put out 120F (at 10GPM). But I've yet to find anything that confirms this.