I would definitely be interested to see this idea materialize as long as it's just optional, as stated. I strongly agree with redwing that it would be great if each of the changes could be disabled/enabled separately.
- the header bar with the back/ forward buttons, location bar and panel layout button to not display. For the search box it could go into the caption bar. Not sure about any custom toolbar buttons
I overall like this change since that bar is just taking unnecessary space most of the time, if you are not using custom buttons that is. I would appreciate though if the back/forward buttons made it to the caption bar as well, as I find them quite useful for navigation.
- the Now Playing header with the total playlist duration to not display
Good change as the bar seems redundant, for the purpose it serves it just takes unnecessary space. If you can come up with a "clever" way to preserve it's functionality (to display the duration) in a not so immediately visible way it would be a plus (unless someone can point out an already possible method)
- the bottom bar with the open/close buttons and the file summary stats to not display. The operation for opening/closing the panels would need to be done via a menu
I find that selecting files and instantly knowing the duration, size etc of the selection is quite useful. Maybe show this info only when a selection is made? Hm, I'm not sure what the actual implementation should look like though