Just a question out of curiosity, but do you sometimes hear the vocals totally blowing the rest of the orchestra away?
I suppose it is rather vocal heavy for my taste as well, but there are a couple reasons for that. One is Messiah is really a 2 1/2 hour vocal work with some orchestra accompaniment. So it is rather appropriate, or "in character" maybe, to have the choir take over the balance from time to time. There is not too much going on in the orchestra to focus on really. Mostly staccato continuo chords and the occasional string swell. If it were Beethoven 9 or Carmina Burana the balance would certainly be more balanced between the two. In most cases I'd say an 80/20 ratio between mains and spots is what I shoot for. This was more like 60/40.
The second reason is because I know from past experience the director will want it that way. I have found that choral directors disapprove when the orchestra is too prominent. Who knows, maybe he'll change his mind and I'll get to remix it.
I don't do too much in terms of automation. It was only 4 channels, so there is not much possible in terms of changing the natural balance, not that I would try that unless there were major problems. I did turn up the spots about 2dB during the solo voice arias and back down for the choir.