Good airflow is like a wide, deep river. Comes in, goes out, nice and smooth. Doesn't have to be fast, it just has to move well in continuity. That's the design of the typical case, air in front, picks up heat along the way, goes out the back.
If you've ever seen 2 rivers come together, it's a mess. Nothing but turbulence, water flying every which way, shifting currents, eddys, undertow etc. That's generally what happens when putting too many fans in odd places. A side intake might seem like a good idea, might even drop the gpu a few °C at loads, but the end result is a circulatory airflow that's dumping/pushing gpu heat back towards the intakes. This raises case temps over time, which ends up affecting the cpu, Northbridge, Sata chipsets, drives etc.
Yup. Flow is king, volume is a distant second. Case airflow is difficult to theorize given the turbulence from fans and the compressible nature of air. Technically, you need the mass of air to act as thermal mass, but since a) air is compressible, and b) density is a function of temperature, it's really a mess to say the least. Volume is easier to discuss since it's a given with X amount of space inside the case, and Y airflow at any given time.
As such, top intake is unorthodox; it disrupts the traditional front to back airflow, and only to be used in cases (heh) where front intake is suboptimal. A case with good mesh front, or at least an inch of clearance from the front of the fan to the solid front panel is enough. Top intake isn't inherently bad, it can be made to work especially if there's a good path for airflow towards the bottom, in theory. Its close proximity to the hot components can make it work, but with no good flow for the air from the top to go, it's difficult to justify.
Furthermore, generally, exhaust is better than intake for removing heat. It's bad for dust prevention, but with
some airflow (discounting tiny vents or gaps in panels), exhaust is still better for removing heat. You can get much more mileage out of a good fan as exhaust compared to as intake.
Really, one or two fans with minimal obstruction as intake is good enough. Most of the time even the top fan on a 3-fan front panel is doing nothing. It's just there to give a little bit of flow towards the exhaust at the rear. Basically, the front fans are doing just that; directing flow. With good panel or gap up front, I find it's nearly no difference in temps between bargain bin fans ($2 LED ring fans) up to Corsair SP120 PE (the old ones).
Bit-tech's article also covers this as well, and they're pretty thorough with including chipset ( I guess representing other components) temps as well. I concur with their conclusion of leaving top as exhaust at the rear mount, and leave the front mount open (or seal it if you can). Adding intake at front top directly disrupts intake from front fans, and top rear as intake makes little sense in most use cases.
To add a bit: side panel intakes are similarly on edge case use. If you have poor airflow near the GPU area, then yeah, it'll help. If you already have good flow from the front, then there's zero use for it. I had a case with 3x front fans that I replaced the front and side panel with custom acrylic panels. The one in front has cutout for the 3 fans so no obstruction from the panel itself, and the side panel has 2x 120mm mounts with holes for ventilation. I found that the side fans do nothing at all, in intake or exhaust. There's barely a degree of difference between no fans, fans as intake, and fans as exhaust. FWIW, one side fan towards the rear as exhaust is good for a degree C that is consistent in my tests.
Good for old cases with a single intake: 1 120mm fan that's also cooling harddisk or what have you, not so useful for recent cases with modern sensibilities.