The restriction is to build coolant pressure in the block which prevents the coolant to bubble and cavitation around the top of the cylinders and heads. This is a well known fact that you need pressure in the block to prevent this. A thermostat can fail which is why many eliminate them and stats are not used in a 500 mile race car or any race motor for that fact.
But you need to restrict the flow so the motor does not cavatate. I don't care what your temp gauge reads If the coolant is not around the sensor it will read a lower temperature. Run a motor without coolant the temp guage will not read hot air. Use a infared thermometer and you will see it is hot at the top of the block but the gauge reads colder. Idling with out a re-stricter and no stat causes cavatation which causes in correct readings. The motor must have a stat or a re-stricter to keep the block pressure up.
I better tell the NASCAR guys that I know your logic and most all racers . They seam to be doing it all wrong. Cavitation is caused more at idle from sonic waves from the block. Before the braided hoses were used they all ran was a 1 inch re-stricter. Now they all have a 1 inch inside hose and fittings. Again restricting the flow keeps the block coolant under pressure. and cools the block and heads evenly with no cavitation. When the top of the block around the cylinders don't have water. you will distort the cylinders. and over heat the heads.
Read up on how to prevent cavitation at idle. Ah but any info but yours is false. Because you know. I didn't come up with this as you did. I just followed the expert advice from many I trust that use this info.
Some info I pulled. There is more out there But it would just be a waist of time as always with you. running hot at idle is caused by to great of flow. No coolant pressure in the block
Engine blocks machined with limited or no cooling jacket can result in steam pockets and hot spots. Boring is great for getting more power out of your engine but notorious for contributing to overheating. At idle the creation of a hot spot at the top of the cylinder is greater and may be enough to cause pre-ignition. Steam pockets can lead to detonation (hot spots in the cylinder wall) and detonation leads to broken parts. At high rpm the coolant moves through the block fast enough to prevent any steam pockets from forming. At idle if it does not raise engine block pressure 22%. Which this helps prevent their formation of a steam pockets and suppress es engine hot spots caused by them.
Overheating at idle can be caused by to great of flow by no restriction at all. This is just something the PO can look into . I am not saying this is his problem . But I always used this method. I have tried down to 5/8" and the best was 1" as suggested by the NASCAR shop for my motor.
I ran pump gas in my motor with 12.5 pistons on the street and never had detonation. At the track when hooked up with slicks I did run 114. I have taken my motor way past 8000 on the street for about 8 year s. NASCAR is using this same method and see 9000 rpm's consistently for 500 miles as stated. I would take their advice to the bank over an assumption from some that have only their experience.
Cavitation can be the OP's problem, Just a suggestion. Steve
