The coolant needs to spend time in the radiator in order for it to cool off.
I run my race car without a thermostat (Valve that opens at a certain temperature, every car has one). If I don't put in a restrictor plate in place of it, the car will overheat constantly for the exact same reason. The plate is no small thing either, it takes a 2" diameter flow down to a hole the size of a nickel.
Seriously, unless you can quantify that first statement somehow, that's one of the most abused myths in watercooling.
And on the topic of your car, are you sure that your engine doesn't have poor coolant distribution? I know some older blocks are notorious for the coolant "missing" certain areas of the block, unless you restrict the coolant's exit from the block somehow. I know at one point, they were experimenting with lower coolant flow rates in NASCAR, but found that it was a bust.
i7 2600K | ASUS Maximus IV GENE-Z | 580GTX | Corsair DDR3-2133