Oh really? I lost out on a BOS due to my brew being "slightly under carbonated". It was the only "ding" against it.
How do you know, did you witness the BOS? Generally no notes are kept from the BOS table, at least not in our neck of the woods.
He could have watched it. Lots of BoS panels are held in front of an audience.
Also, BoS panels can turn into endurance contests. It's possible that the beer had the right level of CO2 to start with, but had gone flat 45 minutes later.
Lower than expected carbonation can cause a whole host of dings - appearance, aroma, flavor and mouthfeel can all be affected. In fact I often serve a nearly flat beer during the BJCP exam and the major flaw, carbonation, is always noted, but many other things can be underwhelming and notable when the carbonation level is too low.
Poor carbonation will mess up aroma perception because less stuff is outgassed, it will make flavor seem sweeter because of lack of CO2 "bite" and will make body seem heavier for the same reason. The surest way to turn a good beer from a contender to an also-ran is to undercarbonate it.
My guess for CO2 volumes is that any beer that the BJCP describes as having "low" carbonation should have less than 2 volumes of pressure. Anything with "medium" carbonation gets 2-2.5 and anything with high carbonation gets 2.5-3.0+ volumes.
As others have said, there are tables which list CO2 volumes for specific styles. Books and websites on commercial beer draft systems should have the info you need.