The issue is not the many leaf objects for trees or the size of the file. I can handle this nicely with hiding the heavy trees in layers. Modelling like this in SketchUp is fine for me.
Swapping heavy objects with proxies, just because I want to open up the outliner to arrange some objects and then close it again and then swapping back, is a bit combersome.
If I would use FredoGhost to do this, then in fact I would generate even more objects, as you then have the proxy and the real object in the model. So making it not better.
It is not primarily the heavy objects, that cause issues with the outliner, but the number of items in the top level shown in the outliner. So if you have a 100 objects in the top level, you already have issues, because you cannot further collapse the top level items.
Therefor a feature for hiding objects from the outliner would be a simple method to imrpove its performance. Same as in SketchUp you hide objects from the viewport to improve its performance.