Thanks for the clarification Dash. It is hard to find information, but I try, and the board here is most valuable in that respect.
The th8000 seems to work fine (you can hammer a nail with a wrench also), it just it does not crontrol the variable speed like the IAQ model will... the IAQ will overcool if >5% hum difference from setpoint and if within 5% it will run 80% speed to cool, and then go full speed if needed to maintain temperature. I'm sure it has all kinds of algorithms for hysteresis, trending, timers, safety's, blah blah... but thats the short of it from what I understand.
It has to be connected to the speed controller of the air handler (like obxtech says) whereas the th8000 just snaps on the wall... badda-bing.. onto next customer.
Come to think of it my dad does have the infinity controllers because he hates changing the $40+ filters he uses.. but he spent more on his A/C systems than I did on my last new car, er... last 3 new cars combined (thats 2 passats and one audi). And, the guy he used to install them is now retired... at 45. So I'm hesitant to go full throttle...