We'd expect the EF 200-400mm f/4L IS USM Extender 1.4x to be most widely used on Canon's EOS-1D series full frame and APS-H bodies, but we can also see users pairing it up with high end APS-C cameras such as the EOS 7D for extra reach. The test results on APS-C without the Extender in place are very impressive indeed - there's simply nothing to criticise at all.
Sharpness results are exceptional - the 200-400mm gives superb results right across the frame at all focal lengths even when shot wide open. In principle the very best results are obtained at F5.6, but image quality stays very high on stopping down to F11. Not surprisingly though, things get very soft due to diffraction when the lens is stopped down to F32 on APS-C.
Lateral chromatic aberration is very low. There's a little green/magenta fringing towards the edge of the frame at each end of the zoom, but you'll have to look very closely to see it.
Vignetting is negligible, as usual for a full-frame lens used on APS-C.