At a certain point, the 25X airflow becomes a noise and possibly cost problem: I mean, sure, high CFM fans are cheap, but 25 is a dam big multiplier.

If the profit margins on mining had stayed at the (clearly ridiculous, but a guy can dream) levels of $30/BTC, then it would almost be worth it to cool it with flourinert. However, 3M wants... a lot for that stuff. A lot a lot. You could make it nearly silent. There is at least one company I read about specializing in data centre cooling with flourinert, and the savings in cooling power is impressive. You have to remember, of course, that it depends on scale: What is easy to aircool in your house might not be easy (or possible) to aircool if you have, 5, 15, or 50 of them.