I spent a rather dumb amount of time recently trying to get my MacBook Pro to display more than 1920x1440 on my external 27" monitor. At that resolution, I'm just not taking very good advantage of the size of the monitor. In FileMaker (and other apps) having the higher resolution allows you to deal with multiple windows and palette windows more effectively.
It turned out that my monitor simply wasn't capable of displaying anything higher than that resolution.
I decided to get rid of my old monitor and purchased an Asus monitor capable of displaying 2560x1440. It was a little expensive, but now I'm really enjoying the additional screen real estate, particularly in FileMaker when I'm using the debugger and data viewer.