It would be great if UMG would refresh itself after every change of appearance-related settings in widget. It would cut the time needed to polish UI design.
I understand that sometimes refreshing hang editor for a second. So you could add editor option “Refresh UMG preview after change” 
Plus, would be nice to expose SynchronizeProperties() to blueprints, so UI developers would have extra control.
I did already, as part of my UI plugin. It’s saving me a lot of time if you can see real look&feel of my UI instantly. And I feel that many developers would be happy to see it’s as part of vanilla UE4 
My implementation follows this tutorial.