Just wondering and a suggestion, is there any reason or can you keep the retina resolution on the retina the same. But when you move to a non-retina monitor, simply leave the resolution but resize the vm to a smaller size. It will be blurry, but at least usable vs having the non-retina screen have everything at 200%. Just an idea.. To see what I mean have Parallels open on the rMBP and drag slightly to the external monitor (just an edge) before Mac decides to switch out the window. See how it doesn't look so bad? Just an idea
