It should, though with Arma/DayZ it is different since it is using an old engine, and with Dayz it wasn't intended for this kind of game, it is poorly optimized.
In general GTX 970 sli will outperform a GTX 980, however for some games sli simply doesn't work or doesn't make a difference untill the sli profile gets published.
It comes down to what resolution you are playing at, if you play at 1080p a single GTX 970 is already overkill.