If your numbs are correct then the water pump is under driven and that is bad. Especially in a relatively low RPM street car.
Think about it. If it’s a hundred degree day and you are sitting in traffic, your water pump and fan if you are using a mechanical fan are turning LESS than idle RPM. You are roughly 20% under driven.
At a 1k idle the pump and fan are now turning 800 RPM. That makes it hard for any cooling system to keep up.
Those stupid under drive pulley’s were all the rage in the 1990’s, especially the circle track guys who though saving 7-8 HP by slowing down the pump and fan was a good deal. Except they lost 20 HP when they couldn’t control the engine temp, especially during a caution.
And there is always a caution when you turn left all the time. And you could easily tell who was running those pulleys. They were the cars that on restart dropped back several places until they had enough pump and fan speed to cool the engine down.
A 20 degree increase in engine temperature is a big enough power loss, but you also have to add in the fact that most of these cars were on the edge anyway, and the increase in coolant temp would drive them right into detonation. If they weren’t careful they’d be spitting rods out before they finished the main.