I have been experimenting with this the last two days. Right now I am using the expand mode so I can use my middle monitors 2560x1440 res. The two outside monitors are 1680x1050. There are some games that the hud or some features are lost because you are losing that part of the screen. The nice thing is that on the games that have the hud etc on the middle monitor everything is great. I am ok if I lose the 400 res on the bottom of the outside monitors. For fps your using the outside monitors for peripheral vision anyway.
I used to have 3 1680x1050 monitors but on the games that eyefinity did not work or was poor I wanted a much bigger screen. I got a U2711 2560x1440 screen about 2.5 yrs ago. This is one of the reasons that I have not been on WSGF for awhile as I have been playing on that screen.
When I would try to play eyefinity it would then default or "fill" to the 1680x1050 screens. It worked ok but now my nice 2560x1440 middle screen was at 1680x1050 scaled. So the most important part of my vision while gaming was at 1680x1050 instead of 2560x1440. I saw more in total width wise at 5040x1050 but gave up a lot of res in the middle screen where res is the most important and 400 or so res in height.
Going back to the new expand mode> when I am gaming at 5920x1440- {1680x1050--2560x1440--1680x1050} The middle res is 2560x1440 and looks exactly the same as when I just use the single screen with the same res. The big issue on expand is game huds or for example some fps game where you pick a gun class or loadout that is on the outside monitor either on the upper or lower part of the screen. You may not be able to see anything at all and the game is unusable even though the game is ok once you are actually playing.
Now on to the new FIT mode. In the video it shows a 1600p monitor with the top not displayed matching up in a nice straight line with the 1080p monitor. The issue I am having is with my 2560x1440 monitor is that the dpi is much higher on that screen than the 1680x1050 monitors on the outside. For example and I think this is very important:: My 1680x1050 monitors have 11.75 inches of vertical viewable screen. But my 2560x1440 is 13.25 viewable height. Only 1.5 inches diff. in viewable height. BUT there is 390 res difference. So if you do a fit setup my middle monitor has a narrower viewable area then the side monitors. It is taking from the top AND the bottom of the middle screen because 1050 res is way different at 110 ppi than say 85.
I have said a lot here but these are some of the issues I see currently.
To be honest this will be all for naught when this comes out. I am getting one of these for sure.
http://pcmonitors.info/lg/lg-34um95-219-but-bigger