I know how it works; that's not my point. I'm talking about heat soak. Let's break it down, shall we?
On a typical GM car, you have, let's say, ~10' (3M for you Canadians) of 10-12ga, wire feeding the ignition switch, and, from there, ~5-6 feet of 10-12ga. wire to the starter solenoid. Installing said Ford "solenoid" (it's really just a relay) removes the purple wire from the starter, and now triggers the Ford relay. A jumper is installed from the 'S' terminal to the 'BATT' post. The 'BATT' post also now only receives power when the relay is engaged. The starter motor and solenoid are still subject to the exact same amount of radiant heat from its mounting position. The only thing to be gained here is, possibly, greater longevity of the starter solenoid due to the removal of resistance in the purple wire. The solenoid now has full battery power because of the relay installation. So, where does this heat magically disappear to?