Ooh, and it managed to peak at 5.4GB before that command finished and I cleared the cache by issuing another command. Now it's down to 3.1GB.
I have been to the very brink and returned!
Though, uh... most of the files I wanted to delete, I couldn't because of friggin' permissions issues. *bangs head on desk*
It went through a five-and-a-half-minute period of ~25% CPU and "Not Responding"... and then finally came out the other end, but it's still chewing on moving a large number of files, slowly growing, back to 5.16GB in the time I've spent writing this.
There's Got To Be An Easier Way!™
(Yeah, boot off a recovery thumb-drive and nuke the whole damn folder from orbit.)