Our team decided to try out the Windows Beta version of SourceTree. You are correct that it is much faster. However, we do not like that you removed the bookmarks repository sidebar. That "space" you saved significantly reduced our teams effectiveness and efficiency with SourceTree. The way our projects are organized they live across several repositories - so working on a single item in a Full-Stack environment may mean we need to work with several repositories. It is very hard to do so with the "New Tab tab".
As a suggestion - put it back as an option. Even better would be a collapsible option so that you could have a side navigation panel that collapsed as needed but that auto-expanded when you mouse over it (and could be pinned if desired). But even if that is too fancy, at least have an option that a user could turn it on or off manually. Let the user decide how to best use the horizontal space.