I've used El Cap Photos in the public beta (not the GM yet), and I've seen some video of extensions in it at work (MacPhun's). What I am interested to know is how these extensions work with RAW. If I use Extension X does it work on an Apple RAW rendering, or does X do its own rendering? Since the X-edited photo goes back to Photos, where it seems to be fully editable, and appears to act as the original, it would seem that X is only making parametric changes to a RAW master, ones that Photos can use. Which would seem to mean it's using Apple's RAW conversion, not its own. I haven't been able to find any extensions to use with the beta; although MacPhun has video demos they haven't been released apparently.