tbh i didn't test it (Iam curious now @OT), but i doubt that a small 120mm radiator can cool better than a 2slot air cooler (or even 3slot) therefore an air solution should be quieter, this 120mm rad would be pushing not all the 300w away (a bit will be going away on other places in that cooling config) but doing the major part to cool the water down, due to the small size the rad should be real quick, pretty toasty too.
maybe your thick radiator is helping very much by combating this with more surface area and more water in it.
but yes "why not" is a fair question, somebody should test that. lol.
Sorry, but you're questioning well established and well researched data here. In the case of my Fury X, the stock AIO cooled the GPU, VRAM and VRM, so every major heat-outputting component. Of course some of that heat sinks into the PCB and is dissipated through convection etc (there's metal-to-metal contact between every heat producing component and the voltage planes in the PCB, after all), but that amount is negligible - and the same is true for air coolers, after all.
An example: The Sapphire R9 Fury Tri-X OC is the cut-down variant of the full-fat Fury X with a beefy triple-slot triple-fan cooler that has a partial flow-through design (one fan blows straight through the cooler, another is partially open as well). Overall it has a very good cooler design, though some current designs might beat it due to having 6 rather than 5 heatpipes. This wouldn't be massive change though.
TPU measured it at an average gaming power draw of 230W compared to their measurement of the Fury X (water cooled reference) at 246W. So how does it fare thermally?
72°C under load vs. 60 for the Fury X. TPU didn't measure fan speeds, but
the Tri-X at those higher thermals (despite its lower power draw) produced the same (very low) amount of noise as the Fury X. So, a three-slot tripel-fan air cooler can't keep up with a single - admittedly somewhat thick, but still - 120mm radiator water cooler.
As for the radiator getting 'pretty toasty' - well, the GPU is at 50-52°C, so it's physically impossible for the water or radiator to be hotter than that (if it was, it would be warming the GPU to match). It's likely to be lower - see the part below about specific heat. CLCs/AIOs do tend to allow their liquid temperatures to go higher than many water cooling enthusiasts do (there's a lot of quasi-superstition about water temperatures in water cooling circles, and a lot of arbitrary lines drawn), but that's not a problem as long as the system is able to cool the components. Which they clearly are.
One thing you're clearly forgetting here: fan size. Most GPUs have ~90mm fans - some smaller, some larger. There's a tendency towards larger today, but they're still rarely above 100mm. They are also very, very thin - typically 10mm or less. That makes these fans pretty terrible overall. There's a reason why GPU deshrouding is so effective - larger fans are simply better.
Just compare a Noctua NF-A9x14 to an NF-A12x25 (I would choose a thinner fan if I could, but there aren't really any available - nobody makes case fans that thin, as they would be terrible): in an ideal, unrestricted scenario the A9 produces 50,5 m³/h of airflow at 2200rpm, while the A12 produces 84,5 m³/h. That's a 67% increase in airflow. And unlike the A9x14, the A12x25 is specifically geared towards static pressure, i.e. it is not particularly good at producing lots of airflow in an unrestricted environment, but rather aims to overcome restrictions such as radiators. As such, it can produce a static pressure (when completely restricted) of 2,34mmH₂O, vs. 1,64 for the A9 - and that's including that smaller fans have an inherent pressure advantage (due to there being less potential open space between the rim and hub, so less area for back-pressure to work on). Another interesting comparison is the A12x15, which is a more airflow-oriented design: at 1850rpm it can produce up to 94,2 m³/h, or 1,53mmH₂O.
Now, GPU fans often run up to 3000rpm or higher, but that's very noisy, and they do so as they can't move sufficient air otherwise - they're simply too small and thin. It's also worth mentioning that GPU air coolers are a very restrictive scenario (blowing through a relatively thick fin stack backed by a PCB, meaning the air needs to do a pretty sharp 90° turn to escape, which requires pressure to ensure flow), which is doubly disadvantageous for these very thin fans. And fitting a 120x25mm fan on a GPU isn't really feasible within stock PCIe slot measurements. That's why people deshroud.
In addition to this, water can soak up a lot more thermal energy than metal before its temperature rises (its specific heat is nearly 4x that of aluminium), meaning for water to increase in temperature by X degrees takes 4x the thermal energy as it would for aluminium to rise to the same temperature. So if you have water at 50°C and aluminium at 50°C, the former is holding 4x the thermal energy of the latter. Aluminium heatsinks thus have a far greater need to vent their heat (they can't hold it without getting really hot), meaning they reach thermal equilibrium with the heat source much faster and likely at a higher temperature. This is of course dependent on the water cooler having a decent flow rate, a decent cold plate and a decent radiator to dissipate its heat output, but due to the higher specific heat of water there is a lot more leeway.
So, to sum up, water cooling a GPU vs. CPU has the advantage of direct die contact and much more evenly spread heat (allowing for GPU cold plates to do their job much more easily). Water cooling a GPU vs. air cooling a GPU has the advantage of larger and better designed fans with
far higher airflow - I wouldn't be surprised if a single good-quality 120mm radiator fan at ~2000rpm with a relatively standard AIO radiator produced more airflow than three typical small GPU fans through their heatsink at the same rpm - and the much higher specific heat of water meaning the system has much more time to dissipate said heat before it loses its ability to keep the GPU cool.