Aside from the title you play, I can speak with experience that it's a smart move. I went from two GTX 880 (non TI but factory overclocked so closed to base TI performance) in SLI to a single 980 TI. From benchmarks, the 980 TI is "just a tad" slower than the 880's in SLI at 2560x1440. Actual gameplay is much better, smoother even with things maxed out. Heat is much less so I run the fans at a lower speed profile. Less heat is more of an added bonus and not so much a driver for me.
 
I would think the 980ti is better suited for the higher res. However, if you're already playing with everything maxed out and get a avg low of 45 fps then why upgrade? The upcoming acer monitor is going to be some where around the 1k US dollar mark. (possibly more since the freesync version is around that) and your graphics is already maxed so the only thing you gain is screen res. 
 

Well let me chime in here, I have the LG34UM95(man I wish I would have waited for an announced G_Sync monitor :() and I have 2x980ti's....  Now, some games you will be fine, but with 3440x1440, that's about 40% more pixels.  I believe its around the 5mil mark or so.  Its really good for in-between 2560 and 4k.  Now, with G-Sync, you will have a great advantage over myself, but with that said, on some of the more demanding games, with everything maxed out for full graphic fidelity, I wasn't getting a full 60 fps.  Now, depending on the game, I set triple buffer on and v sync to adaptive and just run with it, and for the most part I get 60 fps on everything I've played so far.  
What sux is some games SLI profile really blows right this second for the 980 ti, but it will improve.  I think you at least want 1 980ti and OC it.  I'm thinking with G-Sync, you should be OK on a lot of things.  Either way, you will be really happy, but I do think if you want to crank every detail to the max, and run full rez, you may want to look into SLI 980ti's....  But I've never used G-sync yet, so I don't know how much it can make up for.  Hope it helps somewhat :)

I was waiting specifically for the Ti. Going to do some gaming tonight and pay more attention to my FPS and see what I'm doing. I know one area where it just kills frames it's so full of people. Either way, the monitor will happen, even if I take a slight performance hit. I was just trying to see how bad if one.

I can't speak on your monitor setup, but I use the 2560x1600 monitor and the 980 Ti is plenty good for me and I max everything I can.  I came from 2x 680s vanilla versions though.

Keep one 680 as a backup or sell them. The 980ti SLI is absolutely rock solid on 2K as well as 4K. 

Current rig at 2560x1440 gets me 60-80FPS in FFXIV with DX11 on average. The lowest is 45 but only in one area I have been in so far with 100 being the highest looking at nothing with almost nothing around me. So I should do fine I think with the new Acer with one GTX 680Ti till I save up more for SLI from what I have been gathering from you guys.
Thanks for all the help.