From looking at Process Monitor while scrolling through a test folder with 2500 empty sub-folders, it also looks like Opus only calculates size info for the folders IN VIEW. You can also see this is the case by a slight delay as you scroll where the folder icon is initially normal color then very quickly changes to what the label has defined.
I think that approach also greatly mitigates the potential performance impact concerns. I'm actually going to use this sort of empty folder label filter myself - so thanks for raising up the questions 
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@Leo: while looking at ProcMon I noticed this:
Date & Time: 1/9/2014 4:03:49 PM
Event Class: File System
Operation: QuerySecurityFile
Result: BUFFER OVERFLOW
Path: P:\test\1 - Copy (52)
TID: 1736
Duration: 0.0000041
Information: Owner, DACL
...should we chat on this in a separate topic if this is something to be concerned about? It happens multiple times for each folder being queried in the current view.