Put in another words, lets say you wrote a letter and a bigger restrictor was approved.
Then lets say everyone agreed a 50mm restrictor was a reasonable bump increase. My chair-o-meter gt-power results would guess that you would get a 2.5-4% bump in horsepower with a 50mm restrictor. (Hey wait, this is the same as pulling the restrictor completely). This is the result of a big old honkin inefficient engine that has a very low hp/liter.
So now, SCCA has to approve, manufacture, and distribute a new round of restrictor plates and now you have to run two gaskets. All for what? Wouldn't it be easier to just go out to the garage and pull it off? You would get the same result.