I learned to foil on the larger 145 Blast with the slingshot 76, which is similar in area to the 1500. I haven't ridden the flow foils but they may be a little more high performant? If you already have the 115 and are comfortable with it, seems like it's worth trying.
You may want to move your straps to the inboard position to make it a little easier at first. Not sure about the adjustability of the fanatic foils but you'll need to trim it. I had the mast base about as far aft as it would go.