Macbook Pro 15" mid 2012, external display @ 2560 x 1600 on 4K screen via DisplayPort?
Well looks like I found the answer to my problem.
All I had to do was press the Option button when selecting "Scaled" from the Displays setting; this allowed me to select 3840 x 2160 (30Hz) and 2560 x 1440 (60Hz)



Well, to be more precise, the 2560x1440 resolution is not 60Hz but for a weird reason it shows 59Hz in System Information. Does anyone know how come?