I'm not sure how doable it is but it would be handy if there was some way of upgrading a thumb from EXIF quality to full quality on demand.
Basically I have EXIF thumbs for speed (rather slow decoding full images) but sometimes they're not good enough and I don't want to open the preview pane as that will interfere with the work flow, an upgrade of thumb quality for a specific thumb would be enough.
Ideally a keyboard shortcut could be pressed and it would upgrade the thumb the cursor is over (without affecting your selection), but as a second best a shortcut could upgrade the thumb of the last clicked thumb, obviously ctrl-click twice can be used to select the thumb without interfering with the existing selection.
In my own case I would use it quite a lot with sorting out botanical thumbs, where you have a thousand photos from a trip and need to folderise them from their thumbs. You mostly know what they will be but often the transition from one plant photo to the next isn't clear from the EXIF thumbs.
David