It would be interesting to see karma-per-word, as well, as a kind of succinctness density factor. Although karma points are not equivalent to quality, and you’d need to also factor in average comment length and some other things.
To use myself:
31,273 karma / 351,012 words ≈ 0.0891 karma per word
The submission karma is public, so you should be able to subtract it, but that karma doesn’t seem to be the same as the one for comments (i.e. I think one point in a comment gives you one point overall, but on submissions you need two or three points to earn one in your account).
I'm less interested in the idea as a ranking, and more as a way to evaluate my own writing, with the aim of being as succinct but high quality as possible.
You're assuming "high karma = high quality" which, isn't always correct :) I've had wildly incorrect claims be upvoted a lot, and correct ones downvoted, seems to be more about what the subject is about and what "side you're coming from" rather than anything else sometimes. Other times it goes exactly as expected.
End effect is, I wouldn't rely on karma as a signal for quality, just "agreement at large" or something.