yes for the most part heat load does vary dependant on engine rpm but when designing an engine you must account for extremes like towing a caravan with a load of fat blokes in the back seat. So your car is designed for that. So engine load and engine rpm cannot be taken as being the same you can be at full load at 3000 rpm and at half load at 5000 rpm.

By airflow i mean the airflow through the rad not round the block and 3000rpm doesn't mean 70mph all the time, you could be screaming through town up through the gears so your water pump is designed for everything which means at cruising revs it is well over rated.

As to the thermostat that is there getting in the way of a decent waterflow all the time and what would you rather have control you engine temp as a software person a lump of wax as in the modern car or a nice piece of software.

Also temperature of the engine that has alot to do with not only fuel effiecency but also emissions and liner behaviour etc but basically you need higher temps early on to light up your cat and other things.