"Space makes bass." Big boxes are awesome for music. I think 1 cube for a single SA8 is a great number. It's true that it will diminish power handling some, but Sundown probably recommends a smaller box because people put double or triple rated power to them and wonder why they fail. This is also why Jacob rates his subs so conservatively. If you watch Jacob's 152db with a single SA8 vid, you can see that his box is at least a cube.
Anyone can look at the specs for an SA8 and see that .6 is the "recommended" airspace. Don't just regurgitate what you see everyone else say. You guys didn't even ask him what kind of power he had planned for it.
As far as the aero question, a 3" will be fine for a single SA8. Is more port preferable? Sure. But aeros aren't great for smaller boxes because the lengths get unfavorable. If you go with similar port area with a slot, the lengths will be comparable, but you can make it "L" shaped or whatever you need to do. I'd rather see you do a little more port and go with a slot unless you are set on using an aero, but a 3" is ~ 7 sq in of port and that'll be fine for a daily.
And lastly, the flares on aeros are around 5" in legth by themselves. So, if you need 16.3", cut the tube portion to 11.3". Or just leave it full length (which is 17", iirc). That'll put you between 32-33hz and you won't have to cut your tube. The only problem here is... is any dimension of your box going to be 20"? Or can you make some of the port external? It might get a little hairy fitting a 17" straight port into 1 cube.