I like the 27s myself. I like targets with larger heads.
I did not realize BF4 had this 2nd-screen capability. Do you have to run in Windowed mode to get this functionality?
If so, SLI won't kick in. It runs in full-screen mode only (unless something's changed that I don't know about).
Also, I tried playing BF3 in 3D and found that in a multi-player setting, I was at a massive disadvantage. Now the high refresh rate of my 120Hz monitor has proven advantageous in a 2D setting in BF3. I guess if I stuck with the 3D, I might have gotten used to it. It's just a different feel altogether. Do you really play multi-player BF3 or BF4 (beta) this way?
I was going to swap back to the 24's simply because its the same resolution and I could get 3x monitors for around the same end price, but couldn't give up the 27" lol.
I am in the same boat as you, I tried BF3 in 3D when I got the monitor last year and I was at a disadvantage. It was harder to spot units as everything garbled my vision. If I had stuck with it I am sure I would be fine but I got a headache after about 5 hours so I called it quits on 3D. That plus the obvious FPS decrease caused the game to behave slower, input lag almost.
Just yesterday I messed around with the lightboost hack and played a round of DotA2 and immediately noticed the difference, so much crisper and smoother. I will have to read up on the BF4 second monitor thing but I've heard a few people mention it has that ability.
Update: Sounds like for BF4 its a browser window you have open on your other monitor which displays a full view of the map and updates realtime. Might cause some lag since their website especially takes up so much memory, but it sounds like it runs ok.