There is a clock speed curve that hits the sweet spot for all-core overclocks at the 3900X/3950X @4.2GHz and tails off the more cores you add. If you want absolute per-client performance, jamming that much thermal load into a single PC might not be the greatest option. That said, as noted above, having all of them on one PC means you can take advantage of things like VideoFX that do not work on multi-PC setups.

I would hesitate to bother with non-EK water blocks on it based on my reading a few months ago, and based on my experience with a 3900X I doubt a 360mm radiator is gonna be enough if you actually load the CPU down to 90%+. That said, I highly doubt you will be CPU-bound as it's more likely the GPU will become the bottleneck pretty quickly.

I highly recommend doing some research on coolers including VRM water block support for the motherboard(s) as a 280W TDP CPU is going to stress the power components pretty hard. Hell, the chipset on my measly X570 has its own heat pipe + fan setup (and yeah, it can get pretty hot if you don't factor that in). Thermals are going to be the hardest thing to elegantly control on that system, IMO.