I know 2560x1080 works fine in the other Real Virtuality engine products, so it could be DayZ specific, but lets make sure you are editing the correct profile config.
You should not have to
add fovTop and fovLeft, they are there. For win7 it'll be in
c:\Users\{username}\My Documents\DayZ Other Profiles\{profile name}\ in a file named
{profile name}.DayZProfile .
fovTop is the vertical FOV aspect ratio from top to bottom, fovLeft is horizontal left to right. Your values are correct. If you run another resolution, a quick and dirty way of finding the values is to leave fovTop at .75, and take that multiply it by your 'x' res, divide by your 'y' res.
I run 3840x1024 so my RV engine based configs look like
fovTop=0.75;
fovLeft=2.8125;
I'm haven't looked into what the DayZ FOV slider edits, but in games like arma 1,2 and 3 to reduce the FOV I would change fovTop to .65, making fovLeft 2.4375 (.65 x 3840 / 1024).
Also with 2560x1080, you might need to make sure that triplehead=0. I can't recall if somone I know that recently went to 2560x1080 had to change that, but it affects the HUD scaling and is meant to keep it centered for triplehead users.