I mean, GN has demonstrated 10* differences in CPU temps but for your review:
Did you actually read your own source and take a moment to study what the article says about "obstacles", or more specifically, did you note the different results between the different types of obstacles?
Why is that important here? Because typically, cases that do NOT have filters do NOT have huge gaping, wide open, unrestricted holes through which they push or pull the air through.
Is there a difference when the fan has no obstacles in its path compared to obstacles? Of course! However, as your article noted,
fans do not usually blow into an empty space, but have a filter, grille or radiator in front of or behind them
I assume there are exceptions but I cannot think of any case I have seen designed for use without filters that didn't have some sort of grill similar to the hexagonal grill noted in your article. These grills are there to prevent lawsuits - by keeping little, prying fingers out of the paths of spinning fan blades. They offer little to no dust filtering capability - at least not when clean.
Now, if we note the 31 dBA graphs for the Noctua, as examples, and study the results, we see that fan moves 46.44 m^3/h (cubic meters per hour) of air into empty space. But again, when in use, case fans do NOT blow into empty space. When mounted in a case, they blow through (or draw through) different obstacles. With the hexagonal grill, that number drops significantly to 31.05 m^3/h. But with a decent nylon filter, that value is only a little less at 28.2 m^3/h.
For sure, there is a difference between no filter (31.05 m^3/h) and filter (28.2 m^3/h). But is that really "significant" as you claimed? Umm, no. Just for visualization purposes, a 10 x 12 x 8 foot room is 960 cubic feet or 27.18 m^3/h.
Now for sure, the difference between empty space (no obstacles) and the grill definitely is significant. But I know of no case or user who runs with spinning fan blades exposed to little fingers like that. And if you think the difference between a typical hexagonal grill and the nylon filter is "significant", then you are entitled to your opinion. But I don't see that as significant.
I think we also have to be realistic and use some common sense here, and set some guidelines. First, I am talking about my experiences and I always buy quality cases
from reputable makers for all my personal and our client builds here. These quality cases use quality fans and quality filters, and they are marketed touting their sound suppression and cooling options. Example, Fractal Design cases.
I could be wrong but I believe most readers here who buy their own cases, look for those (at least the cooling) qualities too. No case maker worth their salt is going to design, build, and market their cases in that manner, only to have them fail miserably at being capable of providing adequate cooling.
Do filters impact air flow? Of course! But in a quality case that uses quality fans properly configured
for good air flow, the difference between using filters and non-filtered grills is not that significant. If you need to boost the fan speed a little to compensate for an increase in temps, a quality fan can easily do that while keeping any increase in fan noise negligible. And if you need to add another quality fan to increase air flow, a quality, properly chosen case, will allow that too.
I note two quality fans running at slow (quiet) speeds can move a lot more air than one fan running at high speeds.
If the case you selected and bought does provide adequate cooling quietly, and/or does not support adding another quiet fan, YOU FAILED to do your homework and bought the wrong case for your needs!
That is not the case's or filter's fault!
And for the record, I don't say this much but, I really hate fan noise