I can't say whether or not they accounted for the change in weight distribution an outboard would cause (I certainly hope they did!) but even if they did, in my experience (all of 1 boat) even a little bit of ballast down low enough really does make a difference. I was able to get the extra ballast I put in my boat down into the (hollow) rear of the keel itself, but it did wonders.
Worst case scenario, try it out and see what happens. If you don't like it, you get, at worse, slightly slower sailing for a day, and you're out the cost of a few sandbags.