Hi Rob,
I run a GTX 970 on triple 1080p monitors (5760 x 1080) and it will run most newer games at acceptable framerates (between 30 - 60 fps) when settings are set to "High" or "Medium".
Some games (Shadows of Mordor, Witcher 3) require some settings be lowered to have acceptable framerates, but I believe this more due to VRAM limitations as opposed to raw GPU horsepower. I would expect a GTX 980 TI to probably see a 15-25% increase in performance over what the 970 / 980 can do in a similar setup given the increased clock speeds and VRAM.
For 1440p, the TI is probably still going to do an acceptable job across 3 screens with settings lowered. For 4K, there is not even multi-GPU setups that can do triple monitor gaming acceptably in all cases.
So, I guess the main question is, what type of monitors will you be using? That will decide if a TI is going to get the job done.
My Setup (for reference):
i7 4790k @ 4.6Ghz
Noctua D14 Cooler
MSI z87-GD65 Gaming MoBo
16GB DDR3 GSkill Sniper RAM
Gigabyte GTX 970 Gaming G1
Samsung 840 EVO 256GB SSD
3x Dell UltraSharp UH2311 (23") monitors