2 ASUS GeForce GTX 1080 8GB ROG STRIX *OC Edition* in SLI vs. The *NEW* 1080 Titan X

I'd like to know the opinions from some experts on what my best option here is. Mind you, I'm looking to use these cards for gaming, specifically running a 4K resolution at a *solid* 60fps. I have two 2 ASUS GeForce GTX 1080 8GB ROG STRIX *OC Edition* cards currently on their way, and one of them was purchased by someone else as a gift. They cost $724 each with tax and shipping. The new Titan X was just announced yesterday, and retails for $1200. The specs do not seem to be worth the price tag however. Yes, it's sufficient for running 4k at a stable 60fps, but It looks like my 2 cards in SLI could potentially do the same. The specs of my two cards doubled seems to be considerably superior to the Titan(if it works that way). However, I have some concerns about the actual benefits of SLI from past experience. For one, in order to take advantage of SLI performance, games need to support it. Now, this may be more common than it was 5 years ago. If a game supports SLI, will it ultimately double the resources of a single card? (clock speed, ram etc.) Secondly, How many future AAA titles do you think will support SLI?

If you check the specs, the Titan X does not seem to double the performance of a single GTX1080. I believe I read something like a 20-24% increase. Enough to get 4k @ 60fps, but definitely not worth the $1200 price tag. If i didn't receive one of these cards as a gift, and couldn't sell them on ebay(potentially for a nice profit as they're sold out all over), I wouldn't even be considering it. However, if it's more likely to get me 4k @ 60fps with a majority of games, then I am willing to fork out the extra.

Your thoughts and opinions are appreciated. Thanks.