I think "Custom groups" should be on same level as "All Torrents" group. In my case this change allows me to make sidebar little bit thinner. I think there is no real reason too keep this folder on same level with "Uploads", "Downloads" e.t.c. All groups in "All Torrents" group are statistical dynamic... except this "group" and "Ungrouped torrents" group. Even "Ungouped torrents" group is semi-statistical.

I propose:
1. Move this group on root level near "All Torrents".
2. Move "Ungrouped torrents" group inside of this group and change icon for this group.

This changes give user ability to hide all statistical groups (e.g. I don't care about this groups) and see only his own groups and ungrouped torrents (like ToDo list).