Maybe I'm thinking about this too hard, but why not put the entire ~/Library/Caches directory on the RAMdisk? And why save it at shutdown? They're caches, right? Is there any harm in just letting them get rebuilt when necessary? Sure, the first launch of any app after a restart might be slower, but we've got an SSD to minimize that, and I don't restart very often anyway.

True, thought about this too. But, losing the cache on logout will mean you also loose stuff like your Safari preview thumbnails (it will regenerate them on launch via a relatively slow internet connection).

In case of a SSD, it will diminish the advantage you gain (fast app starts) because of the slower cache rebuilds (also counts for Word font cache, etc.).