Thanks all! I've followed this thread for a few months, and very much appreciate the timely feedback to my query! Most importantly, the thermodynamics works and works well -- in theory and in practice with these small scale "process plants"! I was well into my planning for my electric brewery build when I ran into the ventilation dilemma. In my situation, a proper vent hood, fan, duct work and roof vent could easily run $1500 - $2000, and I would end up with a new roof vent where I really didn't want one. There are certainly a few "economical" solutions around this, but that would have compromised my objective. So Brundog's concept was seen as a potentially BRILLIANT and game changing solution to my (and others') situation!
In this context, the cost delta between 1.5" and 2" is almost trivial, and I was much more interested in confirming (now that there have been some learning curves) whether there was a meaningful performance difference in the 1.5" and 2" systems. I am also blessed with a very high quality, high pressure, cool, plentiful, and low cost water supply (hurray!). So there is no tangible cost or environmental difference in using 6 gph or 15, although I appreciate that this may not be the case for everyone. I was happy to go with a 2" system, if there were no downsides other than cost, if for nothing more than peace of mind and added flexibility if ever needed. From the anecdotal evidence, it appears that 1.5" probably handles most needs quite well, although some have expressed added comfort with the larger system.
So now it's time to forge ahead, stop thinking/planning and start acting. I'll be building around the 2" design for my 20 gal BK rig, and will be delighted to report back with my experiences and results.
The community's ingenuity, and generosity to help others (who they may not even know), is very impressive and most appreciated, especially by those of us lacking some of that ingenuity! Thanks again.