Hi Dave,
I don't actually own any kessils but i have plenty of experience stalking other people's journals 😉
They are rated for 2'x2' footprint, so you could get away with 2 for an island Scape but 3 would give you better coverage. From what i have seen intensity is generally not an issue but the point light source casts heavy shadows so if you want densely planted then 3 should give you what you need.
Someone who actually has some should chime in on the recommended model, most people plump for the a360 on larger tanks. That said, i haven't seen many running either the a160 or a360 at full whack anyway.
Hope this is of some help, a bump if nothing else =D
CMac