Too bad I'm in Cambourne/Cambridge. Would more than happily help just for the fun of it.
From experience, CPU temps won't be much gain compared to a good AIO, unless you're reaching thermal limits.
The main gain is for GPU. Watercooled GPU would allow much, much quieter setup, and boost would normally be limited by power or chip, rather than temperature.
I'm waiting for one of alphacool's GPU AIO, as a radiator with 3 fans at 800-1000 are much more efficient and quieter than the GPU's fans at 1500-3000 rpm.
Owned a XL before the air, and using AIO at the top, as exhaust, side and bottom as intake will return the best temperatures overall.
In truth, missus threatened to thrown my PC from the window as, according to her: "it's a computer, not a Lego. Stop to taking it apart".
Tried every possible combination, no big differences, but the worst was radiator at the top as intake, botton intake and side exhaust. The worst affected was the NVME and the GPU. The only gain was 1 or 2 C for CPU, but as the CPU never goes past 60ish unless on benchmark, not crucial. Got more from the GPU keeping the temperature inside the case lower, and the radiator using slightly warmer air as, at least using that many fans, unless really badly setup, the air won't stay too long inside the case, but a 3C to 5C air reaching the GPU can be the difference between keeping a max boost longer or even a higher boost.
Few sets around reported how many MHz you lose on boost as the temperature increases.
If really going for a custom loop, 2 rads is great, 3 won't bring perceptible temperature gains, unless usinng a combination of very restrictive rads and blocks + poor performing rads + very bad fans, but would allow a lower fan speed, as with more rad area, the fans normally won't have to spin faster than 500-800rpm to achieve optimal performance.