So… covering the venting hole is not bad for the fan? Isn’t it over heating?
It’s lesser of two evils. Ideally you want the hole uncovered, but also no dust in there. If you can’t eliminate dust, then covering the hole is not as bad as leaving it open and getting the fan motor full of dust.
It probably does not matter as much if the fan is not spinning at full speed all the time, as it would get less hot on lower speeds. Most other fans do not have that hole.
Why would you have dust in an air-filtered rack enclosure? 
Because not everyone has filtered rack enclosure.. My server is literally in the great outdoors, in the patio closet, next to the gas powered water heater, on the floor. I blow is off with compressed air once a year. I don’t pamper electronics.
So, for you it would be better to leave the hole open. My servers get full of dust and I sometimes lug them outside and blow the dust out with an air compressor, creating huge clouds of dust. If I had those fans and left the hole open they would probably clog up and fail in a few months.
I clean the dust filter in my AC every 1-2 weeks and the evaporator still gets full of dust (though by the time it gets full, outside gets cold and I can just open a window and not need the AC that much, but I have to clean the dust from the evaporator every Spring).
I try to avoid shutting down a server that has a lot of uptime if I am not planning to clean the dust from it right then. Restarts are OK, but if the fans stop completely, when I turn the server on, I get a large cloud of dust coming out of it.
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Fans are optimized for noise profile at their intended most common speeds. Noctua fans are among the best at speeds like 600-1500RPM. These days there are a lot more decent competitors they trade blows with though often for less money. I’m not at all surprised they don’t perform well at 100% speed. They have an industrial range thats optimized for higher RPM, though I believe only in 120mm and 140mm. Anyway, you need to buy the product for the right purpose. Noctua is overpriced, for sure. But I wouldn’t call them a scam. Their fans are good although expensive, when used for the intended purpose. Though I won’t vouch for their industrial range. Never tried those.
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Well, the specs here: NF-A8 PWM seem to imply that at 2200 RPM they would produce under 18dBA of noise. This is demonstrably false. So… how shall I call this?
Ahh, but at what distance though…? 
I don’t have a room in my house with a noise floor of 18dBA. So yeah, that’s a totally BS claim. I don’t go by their numbers. But they still do well in independent testing. Just not THAT well.
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I mean, I measured lower noise and better airflow (indirectly, via HDD temperatures at steady state) from a stock 7000 max rpm fan running at over 3000 rpm, than A8 at their max.
Of course there could a test be constructed where they outperform any specific metric (like hitting any dbA target by reducing rpm down to zero) but fan needs to move air, and they are very bad at it, while also being bad at being quiet. What they do excel at is marketing and environmental pollution – I don’t’ know what I’m more made at – bullshit specs, horrid performance, or amount of e-waste, plastic, and rubber, that came in the box with a fan (fancy double box with flap, plastic box inside of that box, a bunch of cables I did not ask for, a bunch of plastic inserts I did not care for, a bunch of rubber feet preinstalled that had to be yanked out because they exceed fans external sizes, another plastic bag of screws …) /rant. This product shall not exist.
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It’s just more evidence that you bought the wrong product for your use case. I have a couple of noctua fans and they are amazing up to 80% speed, but above 83% speed they sound super annoying. And those extras are pretty great for a desktop PC build. I definitely like having a few spare low noise adapters and those rubber feet help prevent resonance. Though I definitely agree about the packaging.
It doesn’t surprise me that your other fans are more quiet and push more air at 3000RPM. That’s probably their sweet spot, while the sweet spot of the noctuas is likely at half that RPM. They just weren’t the right buy for this scenario.
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After many builds, I found that the main noise source is the grill in front of or in the back of the fan. If you put the fan in plain air, without anything around it, the noise is pretty low on almost all fans. Having metal grills in front or in the back, makes a lot of noise because of those micro turbulences.
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