Are you having instability issues with gimp 2.9?
I have not installed it yet. I have just been using GIMP 2.8 which came from the Ubuntu repositories. The more I use RT, the more I like it. It is well documented, but honestly, a lot of the features are over my head and there are way more tutorials online for GIMP when I get stuck trying to figure out how to do something.
I posted mainly to see if there was a better way to achieve my goal, which is to selectively recover details in highlights and shadows. The place I send my pictures to get printed only takes 8 bit TIFFs or JPGs.