Nothing is faster deep down wind than a formula board, but it takes a big sail and a lot of practice.
When sailing formula, bigger sails are normally the call for upwind and downwind sailing. Sail size on a slalom board is normally picked for reaching. So in 15 knots, I would rig an 11.0 for maximum downwind speed on formula. Bigger more experienced sailors may choose a 12 or 12.5. In the same conditions, I would rig a 7.5 or 8.4 sail on my iS 111 for reaching. If I really wanted to run deep down wind on the iS at maximum speed, I would have to rig my 9.2 and it still wouldn't run as deep or as fast as the formula 160.
I think a larger iS (122, or 133) would be much better at downwind speed than the 111 and could carry a larger sail (9.5 or 10.0), but they would not match the formula for speed.
There is always a trade off so you can't have everything, that's why many of us have a few boards to cover all of our needs.