Its a 2500k, the thermal output when fully overclocked is going to be around 150W. A single 120mm thick radiator can cool that to around a water delta of 13C. If he went for a 240mm radiator that drop it down to around 6C but 360mm would only be around 4C. There are serious diminishing returns to more radiators and its simply not worth overdoing it.
A dual radiator is certainly worth it if you are building a loop but an AIO can provide that (H100) at less cost. The 2500k just doesn't have a high enough thermal output to really benefit from water, air coolers are much more efficient up to about 150W than water coolers are, because they exchange the heat directly with the air. Its only once you start pushing 180W that it becomes clear water is beneficial, and its also the point where dual radiators are a necessity.
Your absolutely correct if were looking at the same fans
the 120x3 @ 700rpms will equal a 120x1 with 1800 rpms..

Thats a HUGH difference in noise..
A 13C water delta means at full load the water temperature will have risen by 13C over ambient. If you have an ambient of 45C then the water will be at 58C. But this isn't the temperature of the CPU. The water block isn't a perfect conductor of heat, neither is the CPU itself. Its most likely the CPU will be another 20-30C above the water temperature depending on the block you get.
I dont think he will see a 13C water delta on a cpu only loop with a 120x2.
Also the waterblock is a lot.
Depending on the waterblock and how his loop is setup, he can get very nice results.
Your also missing the fact that water traveling at 1gpm (flow) => 300W for every 1C.
The lower you go on flow.. the higher temperature water will become which will increase the equilibrium in your overall system.
I cant stress enough how important this 1gpm flow number is... as its the fastest way to see an improvement until you reached that 1gpm mark.
Also yes u get very bad dimished returns as you approach ambient...
Where do AIO's fall?? less then .25gpm...
Im riding on almost pure ambient.. my water temp in cpu loop isnt even 1.5C over ambient.. however im running an original PA120.3 just for the cpu.
However u want to make sure all your lacking area's are at the edge dimished returns territory, because up to that DR point, you do see improvement.