Paul
Cindy and I joined a group tour to Oaxaca last November.  Due in large part to the "approach" of the group, the birding was not as good as hoped for, but in every other way, it was quite good - culture, food, lodging etc,  we still managed 169 species but with a different approach, that number could have been significantly higher and more importantly with the exception of a wonderful trip to the Manialtepec Lagoon the quality could have been much better,  (Quality meaning views and some of the species not seen).  Perhaps of special interest to you, our only owl was a distant heard only Mottled Owl and we also had only 4 woodpeckers - poor interactions/views.
Friends/acquaintances who are excellent birders were there about a week later - also for around 10 days and had extraordinary birding with far more species, better species and better views/photo ops.  The avifauna is terrific.  Will not be back as other places beckon and time is too short, but I know it would be worthwhile.
Contact me off line if you wish.
Blair Bernson