I've decided to get a 980 Ti and get a second one, probably in a couple of months for SLI. I was all set to get reference cards, just because they're generally cheaper, I like the aesthetic and I read they were better for SLI configs.
Since then though, I've read that reference cards are better for SLI configs with no spacing between cards. I wouldn't consider more than 2-way SLI and my board allows a single slot gap between cards.
I'm also concerned about throttling. Is the 980 Ti cut down enough from the Titan X that I wouldn't have to worry about throttling, provided the airflow in my case is good? I'd definitely be OCing the cards.
Case is a Phanteks Enthoo Primo