I am trying to decide on an external monitor for use with my 2013 15" Macbook Pro to use in clamshell mode. I wanted a 27" 2560 x 1440 display, but several people on these forums keep insisting on running a 4K display at a scaled 1440p. Those of you who use a 4K display with your 15" MacBook Pro, how is the performance at native 4K? How is the performance (and appearance) at 1440p scaled? When I run my internal display at anything less than "Best for Retina" I notice a drop in clarity and a performance hit in terms of the fluidity of system UI (swiping between desktops, mission control, etc.). I need some feedback before I make a decision. Thank you.