Specify custom resolution in XCom

I have the Steam version of XCom Enemy Within, and wish to configure it to use a higher resolution than the highest it gives me (1440 x 900).

I have modified my /Users/<user>/Library/Application Support/Steam/SteamApps/common/XCom-Enemy-Unknown/XCOMData/XComGame/Config/DefaultEngine.ini file to ensure it contains the following:

[SystemSettings] ResX=1920 ResY=1080

But this has had no effect. A similar modification worked for me when I was playing this game on Windows 8.1.

Is it possible to play this game with custom resolutions on OS X, or has this facility been disabled for some reason?

Update:

I've also updated ~/Library/Application Support/Feral Interactive/XCOM Enemy Unknown/XEW/MacInit/XComEngine.ini with the same content, to no avail.