How much difference does hot vs cold air make with regards to an Aprilaire 600 furnace humidifier evaporation rate?
I ran the following tests with an ambient temperature of approximately 70 degrees F.  I measured the (hot) water flowing into the humidifier and the water flowing out of the drain to calculate the % evaporation in each case (there is a reasonable margin of error).  


Furnace running in low stage.  Approximate air temperature coming from register is 120 degrees F.  Humidity was 25% @ 70F.  Measured 24% evaporation rate.
Furnace is off, fan running on low setting. Approximate air tempertaure coming from register is 70 degrees F.   Humidity was 25% @ 70F. Measured 19% evaporation rate.
Same setup as 2, but humidity was ~32% @ 70F.  Measured 14% evaporation rate.


Unfortunately, this indicates a large amount of water being wasted, especially with the heat off.  Aprilaire advertises that it uses 3 gallons per hour with a 0.7 gallon/hour evaporation rate (24%), which matches with my test for #1.