I wouldn't go with FE unless you really don't mind a loud setup (and moderately high temps in addition). The cooler on the FTW3 is really the best part about the whole thing, if you're going to stay on air. It handles a load of heat well with fans at barely audible speeds. I'd imagine these would work well if you do have decent airflow in your case.

Agreed on the hybrid part, if he's willing to go that route. As far as air is concerned, I'm honestly so impressed by the FTW3 cooler that unless you have absolute crap airflow they should still perform better, I'd think. They handle load more similarly to watercooling than any air I've seen to date.

Hybrid or FE.

ericfis
Because of the configuration of my motherboard (MSI X99A SLI Plus) the boards are supposed to be placed right next to each other.

That is only the case if you have three+ cards. If you swap the three 980Tis for two 1080Tis, can't you just leave the middle PCIe empty and improve the air circulation?
I'd avoid an AIO hybrid cooling, either get a proper custom loop or stay with pure air. In this case there's not too much OC to gain with more cooling so I'd stick to air for the easy maintenance.

The HAF-X is a weird case- it's difficult to mount hybrid fans in it. 
 
That said, I even tried running the case open and was still maxing out >91F when benching. Probably because the ACX cards were sandwiched together. 
 
The MSI X99a SLI Plus has odd way of configuring the lanes. I'm not sure how it would assign the lanes with a M.2 NVMe drive with a skipped slot (I also have a 5820k). 
 
The FTW3s are nice cards but I don't think the extra $160 is worth it to me. I'll probably just get the FEs and decide if I want to upgrade their cooling as time goes on.

Should run cooler in your setup
 
crank the case fans to max speed also, if they are not already
 
 

I personally have a SLI setup using GTX 1080 Ti FTW3 with an In Win 509 case (I should probably submit my specs to Mod Rigs...hmmm).  Circulation of air was a concern before installation because airflow in my current case appears very restricted.  There are not a lot of desirable intake openings.  Some street spike looking pattern cut into the side panel as an intake on the that is advised to use as an exhaust.  However I couldn't mount the CPU radiator there because of the case brackets holding my SSDs.  While the front panel of my case has a very limited intake because of glass fascia covering the already limited grill.  However, I did make it work and my cards run fairly cool even with the recirculated air coming off the cards.  It took a little bit of work and some tinkering on my part. 
 
For what its worth, the fans on the FTW3 completely blew my mind on how well it maintains the temps.  The fans are actually pretty quiet as well.  Highest temp I recall seeing on a full load during gaming is 67°C playing Ghost Recon Wildlands at the maximum allowable settings, temporal AA @ 1440p.  I have other games, but Ghost Recon is probably the most demanding title I currently own.
 
CoolGTX and JosephL made a very good point in their first posts.  As did others.  Although, I would implore you to check out how airflow is distributed in your HAF X case.  Try picking up some incense sticks at The Dollar Tree or a similar store.  Light em up while using the smoke as a guide to how well your case pulls/pushes air.  It's like a poor man's PC wind tunnel.  From there, choosing any of the GTX 1080 Ti(s) offered by EVGA is up to you.
 
Hybrids are probably the best option if you have the ability to mount 2+ radiators because most people orient them as exhaust.  If you need more exhaust flow and must sammich the cards due to PCI-E lanes, FE cards would be the go to here.  ICX (Is that the new name now?) are probably the best cooling performing cards but that would be dependent on your airflow circulation.