I am having the same problem as Jan Philip. (I also have the same screen shrinking problem when I try to go to 2560x1080.)
I have a Macbook Air from early 2013, and I am trying to connect it with the B296CL 29" 2560x1080 Display.
The HDMI to minidisplay conversion didn't work, which doesn't seem surprising given what I've seen on the threads. I am now using a display port to mini display port cord because I thought that was the only cable capable of carrying a 2560x1080 signal from my Macbook Air.
SwitchResX doesn't seem to work, but I'm not completely sure I'm using it correctly. (when I toggle my acer display to the appropriate 2560x1080 resolution, I get the exact same problem as when I change it manually through system preferences.
I also tried Keap's solution of changing DP 1.2 to DP 1.1 and then switching the resolution on my computer, but the same problem occurred.
Is there something I'm not trying? Jan Philip, anyone, any solutions?
Any help would be incredibly appreciated. You guys put me to shame in how much more you know about how to deal with this stuff- thank you!
Nick