Yes indeed. The Fury X trades blows with the 980 Ti in terms of performance when both are at or near stock clocks. The 390X is a rebranded 290X with higher core clocks.
http://www.anandtech.com/show/9390/the-amd-radeon-r9-fury-x-review/13
So add a few frames to the 290X and that is roughly the performance of a 390X. Of course, once you overclock it is possible to get more performance out of a custom card. Fury/Fury X have not been stellar in terms of overclockability but are currently the two top offerings from AMD at the moment.
Also, 3440x1440 is about a 35% increase in pixels over 2560x1440, so you won't want to skimp on graphics. Especially when purchasing such a high-end monitor.