"How do you account for the improvement in performance after stopping, and after turning the heat on?"

In my mind, without you telling us different, you stopped at a filling station and put cool fuel in it. Or coincidence if the tank pump is intermittent and the brushes are short.

I've not heard of any performance loss because of a cooling system malfunction, but a loss of performance caused by lean mixture might result in more difficult cooling.

Also, hearing the tank pump working is no assurance the main pump is getting fed, but I'm sure your listening around would have pegged the raucous cavitation symptom that results. I wonder how many times we all wished our EFI cars had a fuel pressure gauge on the instrument panel to augment the coolant temperature gauge.
--
Art Benstein near Baltimore

Just found 2 lumps on my car battery.
Had them tested.
One came back positive.
Hope it's not terminal