Mark, sure the ability to play either fast or slow will enable one to have lots of choice in choosing repertoire. Back to your previous question, the ability to execute technical passage with all physical (in this case rh fingers) combination sure is an excellent start to mastery.
But I'm speaking as an amateur who barely have time to practice. It's almost impossible for me and I believe the majority of us in here to make a dedicated practice to hone technical ability for every physical possibilities/combination. I prefer to use my time to listen a music, choose my own prefered music and finally make an exercise from that, not reversed.
Anyway, I think there's infinite possiblities in composition, so indulging myself in separate exercise like Slurs, scales or arpeggios doesn't guarantee that I wouldn't face many obstacles in future, even if I've already break to virtuosic level IMHO. I still prefer to choose my repertoire first
'The woolen cloth of his suit made me question the value of Chinese silks. His fountain pen made me ashamed of my writing brushes and Chinese writing paper' - Puyi
'I don't write music to be difficult, only to be more like itself' - Harrison Birtwistle