The main thing that mislead people is the peak temperature.
If your CPU or GPU are working as expected, without thermal throttling, the only benefit of more airflow or even water cooling is reach lower temperatures at lower noise levels, more on water cooled, anyway. With air coolers, specially the GPU, if you use the standard fan curve from your GPU, if you increase airflow, not necessarily you'll drop a lot your temperature. The GPU's fans will, most likely, run at a much lower rpm.
Don't know if sounded clear, sorry.
Just as an example, my Suprim X 3080, 380W at full blast, air cooled: almost identical peak temperature using the 5000D, the 7000D and the O11 EVO (with 10 T30 fans now). The main difference is that, with the O11 EVO, the bottom intake does half of the work from the GPU's fans, then the GPU's fans, which are louder by nature, will barely cross 1500 rpm.
For CPU you may see bigger differences, as most people set a fan curve to manage 150-200W, but when at 15-25W, the CPU may be very close to ambient temperature, as long you let it downclock and save energy when possible.
The real benefit of upgrading case is when, after all the possible fan locations (or the ones that actually make sense) have been populated by decent fans (don't need expensive ones, P12/P14 can do easily) and using some common sense regarding intake/exhaust, and yet you're thermal throttling. Remove the side panel, temperature improves considerably? You need better airflow.
Even at full blast the fans are struggling to keep the system cool? The cooling used is either badly mounted or not sufficient.
Setting fan curves is another long talk.