Shop - You said ( I don't know how to copy this boldly like spme people, so excuse me all )
I still lean toward the HVAC system being designed, sized, and installed correctly for its climate. For humid climates I wouldn't go with anything less than a TXV on a single speed, single stage system. It provides far better capacity control and latent heat removal performance than a piston any day.

I would also have to agree there seems to be an aspect of diminishing return on investment for systems much over 13 SEER. The 19 SEER systems with modulating compressors, ERV's, and variable speed blowers sound wonderful and I say if the resident wants that precise level of home comfort, more power to him, especially if comfort is his emphasis over energy savings ROI.

Given the prior discussion on this thread, if the majority of existing systems 13 SEER and below were far better installed and maintained than they are, the level of satisfaction from the public toward our industry would be a lot higher.

ME..... I can;t agree with you more. I too am just trying to learn about the IAQ issues, justifications, and remedies.
And I agree with TXV, and over 13 Seer not worth it. And I agree, if the homeowner wants all that stuff, I will gladly give it to him.
And if money wasn't an issue, I'd have a water-source heat pump with humidity & de-humidity controls, with air-freshened returns, Variable speed drives ( on everything ),
and maybe even an ERV ( or two ).
However, money is 'the' issue around here. And I can't recommend a system to anyone that I would not put in my mothers house.

Muggy -
"If I go with the VisionPro, I'll try the circulation mode and watch the humidity for a while and "report" back and let you know if it's the same, or noticeably worse. Won't be very scientific, but at least it's one data point"

Me... Please let us know, I will be doing the same thing here with a single speed, 10 seer system.. I for one, will look forward to your results. I would believe your figures more than I would the scientific figures.