You're not going to get the same bottom end (light wind performance) on a big iS, as you would on a Formula. Small gain compared to F144. If you want to significantly increase your light air planing time, go Formula.
Like Mark says, it's not impossibly hard for an everyday sailor to ride it well, but on the other side, it's not everybody's cup of tea (that goes for heavyweights too).
Wrt smaller boards (for you, that'll be around 95l), you'll change your mind once you progress and try one. Just wait and see ;-)