Any vapor that is visible on exhale is wasted, therefore, by definition any device that produces visible vapor on exhale is categorically less efficient than one that does not. As such then, the ideal vapor density is exactly that which can be adsorbed by the lungs in a natural/comfortable interval of time -- usually between 3 to 5 seconds. As long as one breathes deeply, lungs are fairly efficient at adsorbtion (for most people, better than 95% uptake in 5 seconds); however, there is a limit, and vapor densities higher than that simply do not achieve as much for the volume of materials consumed.
Given the choice between having something look good vs building something that actually works, Magic-Flight definitely prefer the "works" option. Having visible vapor is also particularly a disadvantage if one is trying to be stealthy. Further, visibility is especially bad since it also implies higher levels of smell as well -- even less stealthy, and in a worse way. Magic-Flight finds it especially ironic to find people asking if the Box can produce visible vapor on exhale -- as if that were a good thing! Magic-Flight worked hard to optimize the Box to make it easy to deliver results without that sort of ephemera.
Also, the degree of visibility of vapor is also dependent on how hot the material was heated (ie, which components are extracted and/or created). Hotter might create more visibility, but this is also not always a good thing since it usually implies either some degree of free condensation (mist) and/or the creation of particulate matter (smoke) -- both of which are bad. True vapor is like a gaseous solution -- it should be clear, and therefore invisible. People who speak of seeing a "milky white vapor" are actually talking about a mist that shares space with a vapor -- a result of changes in temperature more than anything else (like fog at night).