Teri, it's the protein in the egg whites that makes the icing dry and harden. (That's why glaze, or confectioner's icing, takes so long to dry, if it ever does, as it has no egg whites.)
Meringue powder is nothing but dried egg whites and sometimes the following: modified food starch, sugar, gum arabic, calcium sulfate, citric acid, cream of tartar, silicon dioxide, and artificial flavoring. That said, I suspect that adding the (relatively small) quantity of meringue powder you are suggesting will have little impact on drying time, as you're essentially just adding more of the same ingredients already in my recipe, barring the modified food starch, silicone dioxide, and fake flavoring, and not all meringue powder contains these things anyway. If the meringue powder has modified food starch or cornstarch in it, this could lead to faster drying, but again, I doubt in the quantities you mention. I say this because, I have also made 100-percent meringue powder-based royal icing before, and it dries at about the same rate as my icing made with real egg whites. Ambient humidity seems to be the larger variable in drying time.
But, when in doubt, I think the best thing to do is a controlled test! You may prove me wrong, and I would love to hear those results!