When going SLI the lower clock of the reference card won't matter, they'd be fast enough – to put it mildly – for every game anyway. And the reference cooler might really be a good thing in such a setup. On the other hand, there will be aftermarket cards with coolers like this as well – I think I even saw a MSI card which combined such a cooler with a second liquid cooler? Yes, the Sea Hawk: https://msi.com/Graphics-card/GeForce-GTX-1080-SEA-HAWK.html... – of course idle speculation without reviews, but something like this could be cool for you.

I don't think the aftermarket cards are month away. A german vendor was listing some versions yesterday as 3-10 days away. I think that could be accurate, the listings on the vendors sites are all ready.

I would recommend to do it stepwise. One card with two displays, look how it performs, then add one or two displays more and maybe the second 1080, if needed – I don't know which game one would play on more than one 4K displays, and to play on one 4K display one 1080 would be fine.

Don't think I have to explain here that this is still a pretty crazy setup, do I? ;)

The 4 monitors aren't for gaming, they're for keeping code, multiple ssh terminals, file managers, query results, browsers for research and documentation, and all that other stuff up at the same time while working. It's gotten to a point where 2 monitors is simply not enough when there are so many things I want to view side-by-side.

I also figure it'll be a good chance to learn how to offload my data processing to the GPUs with CUDA, and then I'll be able to use as much power as the cards can possibly give, even if it's overkill for any particular game.

I saw the listings for nonreference cards up, but couldn't find indications on availability. Thanks for the advice!