I don't know what an RX-9 is but I've got a .20 R-9 that has counted many coup against feral cats, dogs, groundhogs, skunks, o'possums and small chillun.
For my money, your shot selection was perfect. The head is too iffy because the cat's brain is small and the rounded angles of the skull can deflect the pellet enough to cause it to miss the brain pan. And that close in, if you're scoped, you've got scope height-over-barrel to contend with.
The lungs are a much higher percentage target. Nothing lives if you puncture both lungs. Nothing. It might crawl out of sight first but it's in for a pink and frothy and certain death.
