Have you thought that perhaps you maybe over doing it on the F18 size spinny ? the A class do not have much foward volume and I would suspect that the bouancy will not cope with such a large sail up front. Even if you do go out on the wire the hull will submerge over its length and be consequently slower than just the original A.

Its a real buzz flying the spinny but I'm not sure it will work unless you really back off on sail size and have a fairly full sail to allow you to go much deeper. Even a F16 sail will be a handful unless you expect to sail in very light winds.