I don't think there is a way to change this in 2015, but I use Flat Tree View in part mode. But when I am working with the same parts in an assembly that Flat Tree View is not possible. This kind of sucks since I am using Top Down design and like to see my sketches out of features. See below for screenshots.

If anyone knows a work around, please send it over. If not, Dassault please update!

Peter