I rented the 200-400 and took it to my local airport. With my 1D4 I shot a number of prop and jet planes. Hand held and at various shutter speeds. Most were with the 1.4x engaged. A few were with an external 1.4xIII attached. I was surprised how well some of these shots came out at 784mm.
Note: this is not a lens test. But it does give one an idea of what this lens can do for aviation photography. I'm sure some are better at panning than I, so keep that in mind. Images greater than 560mm were taken with an external 1.4xIII attached.
Bombardier jet 560mm
Cessna 172 600mm