Not sure if this is covered elsewhere or not, but if you're overwriting an existing file (from a parity backed disk), and have a cache drive, what is Unraid's behavior?  From what it LOOKS like on my system, it's actually overwriting the existing file on the storage drives (which is what I would expect), but I just wanted to ask to be sure.

It's going to depend on the application.  If the application opens the exiting file, truncates and/or writes to it, then those updates will take place on the data disk on which the file resides.  If the application deletes the "old" file, and then writes a "new" file, then the new file will get created on the cache disk.