Ok, results are in using uncolored RI.
I made Haniela's Fall leaves cookies. It was also my first attempt at transfers and airbrushing. (It went quite well, i think) It's a rainy day here, but the temp in my house stayed at 74 degrees with a 39% relative humidity all day.
I made a half batch of RI using Julia's recipe. The other half batch was the same except for one thing. I dissolved 2 Tablespoons of CK MERINGUE Powder in the pasteurized egg whites before adding to the sugar and cream of tartar. So, that would be 4 Tablespoons in a full batch using a 2 lb bag 10x powdered sugar.
After 1 hour the regular recipe dented quite easily, and seemed to have more sheen (indicating to me that it's wetter). The meringue icing took a tad bit more pressure to dent.
At 2 hours, it was still a pretty light touch that dented the regular icing, and it still had a bit more sheen. The meringue icing took at least double the pressure to dent. Remember, pressure is subjective since I don't have a pressure meter. 
At 4 hours, the regular icing was still pretty soft, a titch harder to dent and the sheen was now about equal. The meringue icing is a lot harder. Even with three to four times the pressure it stayed firm
At 6 hours, the regular icing still dents with a crusty effect. I'm certain they'll need to sit over night to harden well enough. The meringue icing is ready to be packaged. They're taking a lot of force to dent. The icing is a LOT harder. (Done in my book)
EDIT:
I forgot to mention that the icing made with the added meringue powder seemed fluffier and softer, and seemed to yield more icing, even though I weighed out 1 lb of sugar for each bowl.