Use Apple's FSEvents (and other platform equivalents) to watch for external modifications.

Apple's FSEvents infrastructure will allow Sublime Text to be proactively notified when a file it has open gets modified on disk out from under it, which means that the view can be refreshed as soon as that happens instead of when the view gets focus again. I'm not sure if there are Windows/Linux equivalents to this, but I wouldn't be surprised if there were.