(Veering a bit off-topic here, sorry) Only done night sky a couple of times and chose f8. What do you get at wider apertures? Is the idea to stop the stars streaking?
Exactly. If you wish to get a nice shot where you can see the Milky Way above a nice bit of landscape you want to to have a fast ultra wide angle lens to minimize exposure times and lessen the need to use high ISOs. And in a perfect world this UWA lens would give you really good corner sharpness and no coma aberration (optical aberration distorting the stars in the corner to oblong splotches).
If the 16-36 f4 would deliver that kind of optical quality/sharpness across the whole frame (FF), I would be willing to give up f2.8