Frank if you shoot RAW, and really you should for the added DR, then any of the internal crop modes of the 5DS are redundant. All they actually do is apply a soft crop to the full sized file, which can at times be difficult to get rid of if you change your mind. The RAW file stays the same size, even with the APS-c crop mode you get a full sized RAW file.
Personally I would love a 5DS for airshows, with the same linear resolution as a 7DII just about I could crop out a nice frame filling shot using my new 150-600C when the aircraft are still at a distance (Safety distances at the larger shows here in the UK seem to be quite a bit more than at comparable shows in the US) For closer/larger aircraft though you have 2.56× the sensor area to catch them with. Last Sunday I was at the last UK airshow of the year, at the shuttleworth collection, and it was the last formal public display of the Vulcan bomber. I could only just fit that in the frame at the closest point at 150mm. All that extra room would have been nice, I could probably have kept the focal length longer and filled the full 35mm frame. I would buy or rent a 5DS in a heartbeat but unfortunately the place I rent from is charging £203 for a three day rental, although if you order it right you can have it for five days at that price (or six if you pick a bank holiday weekend). It is a lot too expensive for me though, although I suppose I could convince one of my product photography clients that they need to get some super high res images in the correct week for a show .