People saying that four Titan Blacks will offer better performance obviously don't understand how SLI works.
OP, when it comes to SLI, you're entirely dependent on the developer and Nvidia working together to support and optimise it. If the developer decides not to support SLI, then that's it, game over.
SLI works on a sliding scale; the more GPUs you add, the fewer performance gains you get, and if you buy four GPUs you also need a top-of-the-range motherboard in order to have the correct PCIe slots. That's before you get to the PSU, case and cooling.
The 6600K and SLI 980 combo is the best choice for gaming, no question. That said, if you can have SLI 980 Ti instead, that's what you should go for.
Yes, exactly. You hit it right on the spot. That's what I've been trying to say for awhile now. There are actually quite a few games that don't support two way SLI so there being support for three way SLI and four way SLI isn't all too common.