You touch an issue here, that is inherent to all multicylinder engines with a single carb:
Basically, it is impossible to get an EXACT equal fuel distribution to all cylinders, and this distribution is influenced by many factors, in other words, it is most likely not the same at every RPM. As long as these imbalance is within a certain range, this is no big deal, after all, nothing is "perfect", right?

Fuel/air ratio determines ignitability of the fresh charge, and this is what we adjust with the needles. In a single cylinder, things are obvious: too lean is too lean and too rich is too rich. Not so in a multi: Leaning out, ONE cylinder gets too lean, making the entire engine appear lean, while in reality total average fuel/air ratio could very well be on the rich side. Fattening up that single carb, could very well make ONE cylinder (another one, no doubt) too rich, making the entire engine appear rich,
Usually, distribution is most equal at full throttle (not "perfect", because nothing is, but "most good")

Now what happens when for example at idle cylinder 1 is getting significantly too rich (so you lean out the entire engine at idle to make it run better), but at midrange and higher things even out? Then the engine tends to run lean at midrange at at least ONE cylinder, and again, that makes the entire engine appear to be lean. So you set the LS needle a bit fatter to have a good midrange, and you're back to that rich idle.
And all the while, it is very possible and even very likely, that the "total average" fuel/air ratio is within acceptable limits throughout the entire rev range. The engine is running, after all, isn't it? So at least SOME cylinders should receive an acceptable mixture.
The carb might therefore very well have an overall good fuel linearity, but if the fuel is not equally distributed, the engine is never going to run really well.
So efforts to keep that mixture distributed as evenly as possible, will improve running on ANY multicylinder engine.

The holes only are an attempt to keep the fuel suspended in the intake airstream as good as possible, making fuel distribution as equal as I can get it. They do not serve to correct rich or lean spots. But they definitely have an effect AS IF they correct rich or lean spots. Because the rich and lean spots you observed, are only "apparent", they are not real as far as the carb is concerned, they are caused by unequal fuel distribution. They are NOT an airbleed, and are NOT tunable, because that is not their purpose. Their total cross section as well as their effect on fuel/air ratio per se is negligable.

So, the holes do not serve to tweak the fuel linearity. That is the job of that electronic device that is at the base of the title of this thread. It basically makes that LS and HS needles and their correlation do not really play a role anymore. What we do is set a more or less "random but acceptable" LS setting and HS setting, and once that is done, these needles are left alone. The entire throttle range, fuel is controlled by a curve programmed in the transmitter, and basically we have for each conceivable throttle position a separate needle, in an indefinite number. I can literally set mixture independently for idle, 10% throttle, 20%, 30%, etc etc etc, all the way up to full throttle.
And when the weather changes, the ambient temperature, the barometric pressure, or even the flying altitude, this is corrected automatically.