All I've got are two Corsair ML 140's on the front intake. Whether I go 120mm or 140mm, my case can only take two front fans so 140mm it is. They peak at around 1100 rpm but typically it's more like 500-800. As far as I can tell, more air cannot physically be drawn to the case. It overcomes the restriction of the filter and front panel about as well as possible. If I run the same stress test, one run with those fitted and another without, I see roughly the same temperatures at the same speeds. It's a good balance. Suckers have some pull even at low RPM's and they're pretty quiet!
My case (NZXT S340 Elite) also has a top 140mm and rear 120mm exhaust spot, but I don't populate them. I found that all they were doing was making the front intake work harder to push the same air through and directly contributing to noise themselves. The flow pattern just wasn't right, I think. The exhaust must've been throwing a wrench in it. I had exhaust from the GPU being drawn right back in underneath. CPU cooler was working harder, too. Just removing the exhausts reduced its RPM's a good bit. And that's not to mention the dust being sucked in from god knows where. My setup runs much quieter, holds lower temps, and draws in far less dust without the exhaust fans. But I think the worst part was the nasty resonance the exhausts caused. Just no benefit. I mean, it ran fine with them in and I kind of figured "why not?" but it really was holding things back more than it was helping.
It really does perform great this way. I run a Ryzen 2600 at 4ghz all-core and verrry rarely does it ever break 55C. And by then my CPU cooler (Dark Rock 4) is just *starting* to approach 1000RPM. More typically I'm seeing maxes of 800, over several days running and carrying out all sorts of tasks. Meanwhile my Strix 2060 will boost to nearly 2100mhz barely breaking a sweat. The only two games to ever push it over 70C have been Metro Exodus and SOTTR... and even then it's just barely hitting 70. Most games are lower to mid 60s. The fans there can get up to 1600RPM under heavy strain, but that really isn't loud for that cooler... ~50% of max speed. And that sucker kicks up some watts! Just shy of 200w, in fact. Decent amount of heat coming off of it, but it all makes it out easily.
I've put in a lot of time to getting the best, quietest performance out of this setup, playing with different configurations of different fans, a couple of different CPU coolers, OCD fan-curve tweaking... everything I could think to isolate, I did. I also did my best to find the best performance compromise for my CPU. The boost on the vanilla 2600 is kind of a shame. Very efficient, but it leaves a lot of the performance off of the table for anything that hits multiple cores. So it had to be all-core, which means more heat and power consumption. After extensive fuckery I found the max all-core to be 4.3ghz! But as tempting as it is to run it there just because I'm lucky enough to have one of the few that does it well enough to actually run it there continuously, the heat is too much. I take that voltage headroom and use it for efficiency. It has the headroom and stability to run that 4.3ghz OC, so it also doesn't take as much as your typical 2600 to run lower. I don't fear the heat, but I greatly dislike the screaming dinosaur sounds my computer makes when I run it that way.
4ghz ended up being the ideal point. Past that, the performance gains aren't as significant as they are at any point below it, and the power usage starts to go up at an exponential rate. Not just more voltage, but quite a lot more current when those cores start getting nailed. 4ghz is stable at 1.1ish volts, which as far as I can tell is about the best voltage you can ask for. But to get to 4.1 I'm already getting closer to 1.3v. That combined with the extra current needed results in a 10C temperature increase on the same fan curve. 4.2 is another 5C over that, as well as firmly over 1.3v. 4.3, I'm approaching 1.5v and going over 200w... nuff said. It clearly likes 4ghz best, so that's where it stays!
And then, there's other little things. I've got rubber washers on both sides of the screws for my intake fans, just to help dampen vibrations. And it does seem to help. I also try to keep all of the space around the machine open and keep everything clean to minimize obstruction... and I also have it near the AC exhaust. The GPU was a little tricky. There is a certain RPM range you have to stay out of because it makes a resonant humming sound, so I set the curve so it never lingers there. Just a steep jump at that RPM range. Hueristics help significantly, too. Temperatures are just slightly higher due to the response delay, but they're also more stable... RPM's overall tend to stay a bit lower. And idle-stopping with this card is out of the question... the fans on this card are ridiculously noisy when they start up... not to mention when they first kick-up they go up to very high RPMs before settling down... I mean, when you're going from no load to high load and temperatures jump, makes sense the fans might too. Some cards handle it better than others. But with the Strix, I couldn't have that obnoxious shit randomly bothering me.
I'm at a point where I don't wonder what's best for my setup, fans/cooling-wise. I just feel like there's nothing left to optimize. It does everything I need it to nice and snappy-like and I barely know it's there by the sound. Meanwhile the parts inside are staying (I think) pretty respectably cool. I picked the parts I did partially with efficiency in mind. Right out of the box, both the Ryzen 2600 and many of the RTX 2060 cards are pretty easy to cool. With optimal configurations they really shine in that department. It's really what they're best at - they do well delivering solid performance with really good thermals. I mean, a decent sized tower cooler, two fans, and a chunky, triple-fan GPU cooler get the job done well enough I think.
It's interesting to see how people run their airflow. Everyone has their rules for what works and what doesn't, but honestly I think advice can only take you so far. The only way to know what's best for your case, parts, and cooling solution is to play around with it. There's nothing mystical or arbitrary about it. All you have to do is apply some basic rigor. That's the only way to know. What generally makes sense may not be the best, depending on the case layout, placement in the room, and components inside.