| First of all a restrictor plate used in racing is exactly like a smaller throttle body. 
A NASCAR restrictor goes between the carb and manifold and
because it has a 4 BARREL carb the holes are for each individual throttle plate in a 4 BARREL carb. Since we have 2 larger throttle bodies two smaller throttle bodies are two restrictor plates instead. IT IS EACTLY THE SAME!!!!!!!!!
I can tell you that if they could make each of the holes just 1mm larger and in your logic 4mm, they would hands down win the race if they did not crash. Other types of racing that use single throttle bodies use a plate with simply a circle hole.
The hole does not necessarily need to be smaller than the carb or throttle body being used.
The plate is designed to keep things fair where some racers would simply port the carb or throttle body even less than a MM and give that racer the advantage.
In the case of NASCAR the cars were going to fast and it was too dangerous and the plate limits the possible power , not matter what you do. I guess you do not understand how dramatic making a circle slightly larger is either.....
The formula to figure surface area of a circle is (pi)r^2 The difference of only 10mm at the size of 58mm in a circle comes to 832.10mm more surface area. Yes, that is not a typo.... The surface area of a 58mm circle is 2640.74mm and the surface area of a 48mm circle is 1808.64mm and yes, that makes the difference of 832.10mm of surface area that you think is nothing. I can tell you small details all add up and are what make any race car the winner.
This throttle body size is not little, this is a HUGE piece and will make a big difference in flow.
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