I live near (sometimes under) the flight patterns of an Air Force Base. I take a lot of photos depending on what is happening. My go to lens is a 100-400, which is what I also take to airshows as my main lens. I also have a 600mm and it is much more difficult to find and hold in the frame any aircraft in flight overhead, either handheld or on a Wimberely. If I am off to the side of a runway end, it is easier to use.
With the extender, taking it to 700, for me would be a challenge (overhead). This was taken at 600mm, about 500 yards off the end of a runway:
I would not take my 600 to an airshow where I would be closer and need to be shooting overhead. I would also consider using center point expansion, or all of the focal points when shooting overhead with more than 400mm. With the 100-400, I use a single focus point and I think it would be adequate.
I would stay with 500, and practice before.