Probably 50% of what appears to the eye as obvious spam gets scored in the 0.4 to 4.9 range. We had some that scored around -4.x because it had Habeas headers in it that apparently were not detected as fake.
So we're using default scoring for almost everything, and would rather stay with the defaults as much as possible rather than start hacking at scoring to artificially inflate the scores.
For the most part, bayes is not a factor (I'm hand-running messages with the -D flag to try to see what's going on).
Um... that's a definite mis-statement. Bayes almost certainly IS a factor. Anything "obviously spam" should be getting BAYES_90 or BAYES_99.. if you are getting lots of FN mail that's not scoring high in bayes, you might want to examine your training.
