Thank you for writing to me. You ask great questions. The people who talk about the humidity are the pool people that sell only gas heaters and have not been to a national pool show in 20 years. It is true that humidity (or water in general) is a great source of heat. But the difference of heat output on a 60% humidity day and a 20% humidity day you will never notice. I would estimate that you would loose 5% of the output. There was a period when some manufacturers like Rheem put out a great "low humidity" heater. Basically what they did was double the amount of freon coils to get the most exposure to the air. These days most of the manufacturers to get the most efficiency and BTU's out of their heaters make their coils huge anyway. I have a 100's or maybe even 1000 heat pumps from Texas and Arizona through Nevada and California in some very dry places and my customers still love me. I have many customer responses on the site if you want to e-mail anyone and ask about their personal experiences. I think I have one from Idaho on there too, but if not you can contact someone from Arizona or Nevada.
Let me know if you have any other questions or if you want me to call you.