Intriguing. Looks very nice.
Couple of questions:
can the boards still stack easily with this back to back cooler arrangement?
can someone with some watercooling experience give me an idea of the BOM to cool 12 boards?
12x80 = 960W
It seems like a great idea but I'm worried it will prove expensive when you add in radiators, pumps, fans, reservoirs. Is this going to need £300 investment on top of the cooling blocks?
it honestly all depends on the waterblock price and number of boards your going to get..
Can you translate this into cost per miner? For example "Watercooling a bitburner XX would cost X more /$".
as burnin said earlier, the cooler will be for two bitburners and the price difference between aircooling and watercooling should be at ca. 15euro per board.

simple math.
15 per Board for the watercooler, 280 for the watercooling itself.
the more bitburners you cool, the "cheaper" it gets.

Watercooling is nothing for beginners, if you have no clue about it, leave it. One leak can kill your bitburners instantly! Nice explaination.
I think before burnin didnt test the advantages of watercooling its all speculation. Only when there is a reasonable advantage it would make sense to go that route. So i hope he will test it.
And i wait for an answer from hetzner if they would allow watercooling in their datacentres anyway.