An car engine is designed to run between 180-200 degrees. In my boat it runs 120-130. At the lower temp it will make less power because....well shit I don't really know why but I know it losses efficiency at colder temps. New cars/trucks run much hotter. Fuel distribution sucks when a motor is cold...that's why carbs have choke's, to richen the air:fuel ratio.
I run a valve between the pump and block to control water flow but it doesn't do much good. I can have the thing almost completely shut and it still runs 130 at the river. Pressure regulators are used on some jets to prevent too much water from entering the block and seeping past gaskets. There was a thread on here not too long ago on that topic...may have been in the v-drive forum. do a search