Taking a side panel off might often make paradoxically make the situation worse or improve it less than one might expect because at least with reasonably designed cooling, there's an air flow through the case that helps to move the warm air out of the case faster, and by removing the side panel, that air flow gets disturbed. Same with randomly adding fans in the system - it might seem like the more fans the better, but it's a bit more complicated than that.
Exactly.
Which is why it is very useful to check the airflow by taking the side panel off.
I for example thought my case would be well working. Two intake fans at the bottom front, HDD cage removed so 75% of the two fans aren't covered by anything.
Rear fan at the very top, removed the holed-metal and put in a way less blocking cover.
Big fat CPU cooler, sucking in towards the front, through the cooling block and straight to the rear fan.
PSU has its separate loop, bottom into the PSU, directly out to the rear from inside the PSU.
But it's bad. Like really bad...
Taking off the side panel reduces CPU temps by 15° and GPU by almost 20°c...
Multiple reasons:
1. intake holes are too small. Air only gets in at the bottom 1/3 through slits at the side of the front cover. Opening the front cover reduces temps by 5°c
2. side panel has a sound reduction pad that has the classic pyramid design. It's totally bad for airflow. Better and more modern cases have flat pads. I don't need diffused highs anyway so a thicker, flat one would dampen better anyway
3. the rear is only 120mm. It's simply too small to get rid of the hot air
4. motherboard/slots/rear panel, whatever you call it. The back of the case basically has no holes at all apart from the fan-hole. Better cases either have slits at the top rear or some holes next to the GPU slots etc. so at least some air can go out at the rear