> You can just look at the airflow charts and buy accordingly?

Not with Noctua and the likes, not always at least. These fans are quiet, but they are not made of magic fairy dust. They simply max out at lower flow rates than some of the the noisy, high-RPM fans people replace with them. Those 40 mm Noctuas, apart from voltage options, don't really come in many varieties.

It's a trade-off - you definitely lose the upper end of the device's original temperature range, but if it's stable under your conditions - great. How do you find out? Well, you replace the fans and check if it sizzles or not.

And funnily enough, a LOT of equipment these days isn't made with formal thermal design, they simply throw in high-cfm fans and... check if it sizzles. It usually doesn't because there's plenty of headroom. It's loud, but who cares since it's not meant to sit on your bench.

Source: replaced PSU and chassis fans in countless pieces of network and test & measurement equipment.