So I take what your saying then is it will be the same as granulated sugar?
Basically. I'd have to consult a cook book to be sure, but basically, yes. Can't go strictly off weight, since part of the weight of brown sugar is moisture from the molasses.
What about the Molasses addition? Isn't molasses also a fermentable and would add points to granulated sugar number?
In this amount? No. We're talking a tablespoon or two over five gallons.
You're really not gaining anything by using brown sugar instead of corn sugar. All you're really adding is a bit of uncertainty.
If you want molasses, add molasses to the recipe. Then prime with corn sugar.