As for the balance, all the singers are clearly in front of the instrumental ensemble, as they wouldn't be in a "choral" performance, which necessarily means the individual voices in the chorus movements are more upfront than we're accustomed to hearing.
It's balance really, not placement - I've heard quite a few consort-sized performances of the cantatas live (including Cantus Cölln as well as some more humble groups) and that's not the balance I've heard even though the singers were certainly in front. There are other recordings (although not of the St Matthew yet) which more closely capture that sound. To me there are quite a few moments where it does indeed sound artificial: for example one particular 'seht die Geduld' in the opening chorus (about 3'26") where the alto very much seems to be getting some help (that or just singing too loud - as far as the musical effect goes it doesn't really matter to me which).
When I talk about it not being an opera (and not only not a Wagnerian or Verdian one but not even a Handelian one) I'm not really talking about pacing so much as dramatic projection and 'characterisation'. The first bass has Jesus' words but also has other things to do - singing
Komm, süßes Kreuz and
Mache dich, mein Herze, rein, for example. So identifying that singer with that character can only go so far; beyond that the structure of the piece is undermined. ("What's he doing singing? Isn't he supposed to be dead?"
) The story being told is also something the congregation knew not just blow for blow but word for word, so exaggerating the dramatic delivery of certain moments runs the risk of being redundant: if the Evangelist makes too much of well-known Bible text in recitative, for example.
