It's an old myth Cameron. A million sources for explaining it - here's one decent one, but I'll leave the others for you to look into. https://www.flowkoolerwaterpumps.com/cooling_faq.html The reason for the small delta T through the rad is, I suspect, a decades old radiator that is full of mineral deposits significantly hampering heat transfer perhaps exacerbated by a mechanical fan design that was expecting Swedish summers, not 102F. For the 4 decades or so I've been playing with cars, no t'stat or one stuck open always resulted in over-cooling, never overheating. If your theory were correct I'd get even better heating in the winter with no t'stat or stuck open --- not the 'where'd my heat go' that actually happens when over cooled. But here's the real kicker - if increased flow/velocity through rad = less heat rejection, why are all water pumps set up to increase in speed as engine speed increases?
--
82 242 6.2L coming...; '15 Honda Fit; '16 Subie X-Trek