I've been in on a 1bbl brew, using the Blichman 55-gal beasts and the Blichman burners, running off NG. I think the total volume came in at 37 gallons or so.
What I can say is that we never got a really vigorous boil going - whenever we get around to doing this again, I suspect we'll be decking out that kettle in reflectix or something similar to help with this.
But non-vigorous boils aside, we did produce a mighty tasty california common ale. Just ran out of the last of it in the last month or so, and we brewed it right around this time last year!