Don't know about icing, since I only use meringue powder in my icing, but I have used them interchangeably in my cake mixes without problems. I whip up the powder with water to make it nice and frothy. But in my area, the powder is expensive, so I use egg whites mostly.
Note that Wiltonâs meringue powder also has an emulsifier (Gum Arabic) and acidic compounds (citric acid and cream of tarter). It should give you better performance compared to just using dried egg whites.