I boot my rMBP in Windows (w/ Boot Camp), and play Kerbal there (better performance overall). It's silky smooth in 2880x1800 with antialiasing on and all graphics options pretty much maxed out. The only thing that prevents this from being usable is the missing support for scaling UI elements. If all the UI could just be optionally scaled by x2 it would make it playable. The flight controls are ok (those can be scaled already), I'm talking about menus, dialog boxes, especially when constructing ships, but also in flight, like maneuver editing, science, dealing with component options, etc.