My instinct is that the zoom would not be my preference, I feel that I would only use it either fully in or fully out, the mid range would miss out except for rare occasions.
Also I am very happy with my 16mm f2.8, I don't want to loose the speed which this lens provides for astro photography.
I don't want AF because it's a pain having to correct it all the time to get the focal plane where I want it.
I need to look at either Sigma or the Rokinon/other Chinese lenses at 8mm to give me a full circle. I don't completely dismiss the 10.5 but would want to think hard before going that route.
I am surprised how I have found the 16mm fisheye so useful, indispensable even. I bought it out of curiosity and because I wanted 'something different', it's that in spades! It has opened up a whole new vista for my photography in a way my Sigma 10-20 DX never could. I have hopes an 8mm fisheye may take this to another level, especially for garden photography.