I hear that theory bandied about a lot on auto sites. Sounds like an exotic scientific phenomenon, but you will find nothing similar in any engineering heat transfer book (my M.S. thesis was in heat transfer). If adding a flow restrictor improves cooling, it is for another reason like preventing pump cavitation or redirecting the flow, not that the coolant is "moving too fast to cool". As a kid, our 63 Valiant boiled over on the highway. A workman nearby looked at it, saw no flow, removed the T-stat and found it stuck shut, put outlet back without it and we continued with no more over-heating. Said one doesn't need a T-stat in Florida (not totally true).