Just curious, when do you find that running 32mm or 30mm tires are faster and when do you run 34mm tires? Obviously loose sand and really bumpy courses gets the wide tires. But when else? Note, I am a 140lb racer that "enjoys" the wide variety of New England courses.
not a whole lot of cross experience here, but from about 15 years of mtb experience i have found the foloowing:
tire volume is huge, having a wider/taller tire allows you to run lower pressures which increases traction in a straight line, absorb bumps better, but decreases cornering grip at the limit of lower pressures. volume, in my book, is almost more important than tread pattern as far as traction is concerned. tread pattern is important for clearing mud and cornering. tread compound/rubber and tire pressure affects rolling resistance more than tread pattern from what i have found
a wider/taller tire will be slower in thin/watery/soupy mud than a skinny tire that can get down to firm dirt to get it's bite.
like you said, really bumpy and deep sandy courses prefer fat tires.
pick width/volume first based upon the course conditions, then choose a tread pattern
I'm in SoCal where it's generally very dry and fast and the courses don't get beat up too much. Normally I run a 30c tire with knobbies on the side and a smooth tread down the middle. Wetter conditions I like a bit more knob. More asphalt/fast paths on the course means more air in the tires.
I generally disagree with the very widespread use of 34c knobbies. They feel much slower to me and they wiggle too much cornering.