I was speaking to one of the older shooters at our club recently and he was telling me that S121 works really well in .45ACP's. He said that S121 is modeled on the American Bullseye which, as the name implies, was designed to suit target shooters using the 1911.
If that's true with a 200gr - 230gr head at +- 850f/s why shouldn't it work as well in a 9mmP with a heavier head?