I do use fermcap, or at least baby gas drops, which are very similar, and it makes a world of difference. That said, I would not hesitate to draw 1 gallon or so off and boil separately on the stove. And I would not worry about hop utilization. Technically you going to lose a little, but it's not like you are boiling a thicker wort, there is just less of it, so the loss is not likely to be noticeable.
Having experienced boil-overs, I recommend using a split boil AND fermcap. You're going to be pretty full even without that extra gallon or so!