Studio Tests (Full frame, with 1.4x Extender in place)

Snap the lens's built-in Extender into place, and the test results are still hugely impressive. Sharpness drops slightly, but not so much that you're likely to really notice, vignetting becomes a bit stronger, and a little pincushion distortion creeps into the image. But this is all basically nit-picking.

Sharpness Sharpness is still very high right across the frame at all focal lengths, and only marginally reduced by the addition of the converter.
Chromatic Aberration Chromatic aberration is slightly increased, and there's a little cyan/magenta fringing at the long end of the zoom.
Vignetting Vignetting is slightly higher with the extender in place, with the falloff pattern intruding further towards the centre of the frame. But at only 1.3 stops in the corner, it's not going to be a huge problem.
Distortion There's visible pincushion distortion at all focal lengths with the extender in place. But it's not especially strong and unlikely to have much of a visual impact on real-world images.