But I never want to overdo it, and also being an organist I somewhat rely on the voices to be clear and audible even without extra volume.
Well, piano playing is something totally different
from playing organ. If you are a good pianist you are not coercibly a good organist and reverse. (My first piano teacher was the assistant organist of the Dome of Cologne, but I can´t say, that he was a real good piano teacher, so I left him after a while. He was very precise and accurate, but he couldn´t teach me a true piano playing, which implies all the differences of touch and voicing! Fortunately I found another piano teacher, who was a concert pianist. From him I could learn true piano playing!) On the piano we have our fingers to create something like "registers" and the "pianoforte"-touch allows us to differenciate dynamic and colour of the tone very directly and individually. So, that´s what makes an artistic piano playing, if you can show, how much you are able to do this. There are not better pieces to do this than fugues by Bach. In piano playing the voicing (also differenciating dynamic range) is absolutely a criterion of quality!
It would be easier to do in a more leisurely tempo, like you usually opt for, but I also highly value excitement and drive (though happily I am most always slower than in my previous WTC recordings).
Of course, I also like excitement and drive, if it´s adequate (and that´s always more or less a matter of taste). But a higher tempo does not exclude a good voicing!
If I choose a higher tempo, than my voicing is usually differenciated in spite of the higher tempo. I many cases today (not in all, really!) I choose a more moderate tempo, because I discover so many elaborated and deep structures in Bachs works. I don´t want to play them superficially! Artistical piano playing for me doesn´t mean to play as fast as possible. (Though I have to admit, that I have had this ambition here and there, when I was younger. But I have found out, that it doesn´t bring anything!)