Engines are designed to run in a specific temp range. As the engine heats up all the metal expands, the gaps designed in are there for that temp range, a lower temp range tightens up those gaps for better or worse.
I agree that modern engines are designed to run hot for emission reasons, and that might be hotter than is good but the modern engines seem to last 2 to 3 times as long as our 60's engines so they must be doing something right. I would go with what the factory used.
