I read somewhere that about 75% of your engines heat goes into the oil (or into the oil first.)
A cooler for the oil is a good idea.
Some Auto-Trans coolers are designed to flow more when hot/less when cool. If this oil cooler is simular in design it would not "over cool". I would think IF it took too much heat away, the thermostat in you coolant would stay closed for a bit longer, and there would be no problem.