Quite risky to run GTX980Ti SLI on a 750W PSU.

Bottleneck or not is totally depend on the game itself, but considering Xeon E3 1231 V3 is a quad core octa thread CPU, it is very hard for a game to fully utilize the resources of it.

I would recommend you to wait for Pascal instead, GTX980Ti isn't cheap now, if you decided to stay with GTX980Ti SLI, wait the price drops after Pascal announcement, it could potentially save you a lot of money.

Are you running 4K resolution?

If your max settings include max MSAA, GameWorks settings (HBAO+, HairWorks etc.) then surely you cannot run at 60 FPS, those settings are FPS eater 

Considering you don't have a SLI ready mobo and PSU, I suggest you to tune down the settings to achieve 60 FPS for the time being while waiting for next gen GPU, even if you get another GTX980Ti, you won't get a correct SLI scaling if the game doesn't support SLI (no SLI profile from games developer and Nvidia), so you end up running the games with single GPU only.

It is always better to run single GPU vs dual/triple/quad GPU, less issue and hassle.

This depends on the price or Pascal, if the price is same like what they are pricing the 900 series then I guess it should be around 2.3 - 2.5k.

Edit: Need to factor in warranty status, card status etc as well.

If you have Ti then don't bother to downgrade from Ti to non-Ti, 1080 is just 25% (average) faster than Titan X and 980Ti, just wait for 1080Ti instead.

1440p (21:9) is still fine for 980Ti, not all games support 21:9 ultra-wide natively so you end up with 2560 x 1440 (16:9) with black border at the side for those games.

Any budget range?

If budget is healthy then can consider getting GSync/FreeSync monitor 

Acer Predator XB241H (RM 1899), 1080p 185Hz TN.

Acer Predator XB281HK (RM 2499), 4K 60Hz TN.

Acer Predator XB271HU (RM 3199), 2K 165Hz IPS.

Asus GSync monitors are overpriced, not worth to get it unless you are an Asus fanboy 

U2515H and XB271HU are under different tiers, one aims for productivity and multimedia, another one aims for gaming industry.

X34 is a 21:9 monitor, as said earlier, not all games do well on 21:9 monitor and the max frequency supported is 100Hz only via overclocking.

I personally won't pick the X34 for the time being because its specifications don't impress me much, paying 4k+ for a curve gaming monitor is a little bit too premium for me 

I am using XB271HU now, bought it below 3k last time when it got released here, 2.9k to be precise if I recall correctly.

I am an IPS fan and it is first IPS monitor to feature 144Hz (165Hz via OC) so it is definitely a must buy for me.

So far so good, everything performs perfectly when paired with 980Ti.

Asus PG279Q is having the similar specs but it is priced at 4k though.

I can speak for Dell UltraSharp series because I own two of them (U2414H and U2417H), it is a premium monitor with ultra thin bezel, high color accuracy and comes with a beautiful IPS display.

For the other 2 models, they are basically the same, just different brands, 144Hz is another thing that you won't want to miss out if you are a gamer and have > 1k budget, once you go for 144Hz, I can guarantee you won't go back to 60Hz easily (99% chance  ), everything feels laggy on 60Hz  .
Yes, but you gonna top up nearly 2k for a GTX1080 assuming you sold the GTX970 for 1k+/-.

Yup, it is worth as well but 1k+/- top up is required as well, preorder price for FE is RM 2299, AIB card should be slightly cheaper, around 2k+.

Basically Pascal is aimed at Kepler or Fermi users, Maxwell is still quite competitive currently, if you are upgrading just for the sake of upgrading, then I suggest you to wait for next gen instead, unless you really do have the budget for this upgrade.