I've never done any tests, but it does seem to make a difference for at least some plants. It's super-arid where I'm living now so I will be putting ALL my plants in humidity trays - once I get some. Plants, that is. There's a ton of discussion about this amongst bonsai growers, and probably African Violet enthusiasts as well.

Im taking the plunge anyway, I know this thread is 2006 and its now 2015......but it might help anyway. I've bought Lime marmalade and a few of the very dark black ones to try. We get torrential topical humidity and rain for weeks on end so if it survives here it will survive in Florida anywhere including Cuba. Im planting in a raised stone bed under a giant Cinnamomum camphora tree so they will get no more than dappled light part of the day. The soil is almost pure black composted leaf mould, moist and free draining. Im so bored with coleus already. They are not dependable being somewhat fragile for me anyway, though as a temp bedding happening they are Ok. I will post the results, could be a matter of weeks even days ): The ones Im trying are recommended by sites selling them for heat and humidity so we shall see. Here they are sold as novelty house plants can you believe, exported by the Netherlands where they are used outdoors of course. Im not sure how they do indoors with aircon myself, I think also a matter of days. One rather nice perennial I did try which has been a huge success has been Ajuga repens in its various forms, all seem to thrive, black scallop is my favourite. They are slightly slow growing in the tropics (a good thing and probably the reason they survive) eventually make a very nice thick healthy ground cover, especially nice in big pots as a ground cover or foil for a specimen plant, sun or shade, the more humidity the more sun. If Heuchera doesn't thrive that's OK its nice there is something with such wonderfully delightful foliage and colour for our Northern cousins in the shade, who desperately try for the tropical look we so take for granted. Im sure these add to their palette hugely, they would if I was going for the tropical look thats for sure.

"This morning outside humidity 99% inside 35% I don't think I have a leak issue." Then think. That seems about right for a 40F temp shift where none of the humidity generated in the house is retained. Leaky houses make for dry houses in winter weather when the temp is a lot higher inside than out. Check the temperatures as well and consult a Psychrometric chart. You might be very surprised. While you are looking for air leaks in the envelope, look for leaks in your ducts. If you have leaks in supplies located outside your house envelope depressurize the home and outside air is sucked in. Leaks in returns outside the house suck outside air in directly and pressurize the living space.

A camera bag is all you need....keep it out of the rain.