2 years ago I fitted a spinaker-system on 2 Prindle 18's. The age of the boats justified experimenting. The result was that both boats are used with spinaker in alle kind of lessons and courses. The boat stabilizes remarkably downwind when using the spinaker.

I also had the opportunity to try the Hobie 16 with spinaker. Also the Hobie is much more stabile with the spinaker. It could be more stabile but the shape of the hulls prevent it from being more forgiving. Same problem is with the Prindle 18.

Don't be afraid of pitchpoling when using the spinaker downwind on the 16. Actually the spinaker compensates the pressure of the mainsail to the mast (specially the top). If you're looking into the way the wind blows in the spinaker it is clear that the spinaker will cause some lifting abilities to the bow of the boat.

Have fun..