When CF works, 980Ti/1080 is not a worthwhile upgrade. If you mine ether for the next 12 months, it will be enough for almost 2 free Vega/Big Pascal GPUs after you sell each of your cards for $100.
https://www.cryptocompare.com/mining/sapphire/radeon-r9-295x2-ethereum-mining/
With all due repect to my good friend RS I humbly disagree unless mining is your forte.
As you can see I have an EVGA GTX 980TI SC in my one rig and 2 R9 290s (I know not 290x but close enough) in my other rig.
My 980TI SC is at 1102 vcore vs stock of 1000 but my 290s are also slightly OC'd at 1000 vs stock of 947.
In Firestrike the graphics scores of the 2 R9 290s exceed the 980TI SC but not by much. Reg 23, 199 vs 18503;Extreme 10,166 vs 8561 and ultra 5018 vs 4233.
In actual gameplay the single 980TI SC is smoother, has more Vram and works on every game at higher texture settings unlike CF or SLI that needs a profile.
SO 980TI is a sidegrade in synthetic benchmark power but works with all games. The GTX1080 has 8G of faster Vram, VR support, much better thermals and is faster than the 980TI.
Frankly, the more I read, the more I am tempted to replace both 290s with a single solid GTX 1080 under water.
The wait for Vega seems way too long.