My current rig includes the Sony a7R III, the FE 100-400mm f/4.5-5.6 GM OSS and the FE 1.4x and 2.0x teleconverters. I've had the bare lens and the 1.4x since September and I've been pleased with both image quality and autofocus performance, with no noticeable slowdown in AF.
I received the FE 2.0x teleconverter last week and was very happy to report that it autofocuses on both the a7R III and a9 bodies, with very little, if any flow down. I thought, holy cow, if I can shoot this thing in good light, I've got 800mm of reach in a very small package.
In early testing, I was very happy to run across a kestrel, in great light, at a reasonably close range, but I was very disappointed that none of my shots were as sharp as I'd come to expect with my bare 100-400m and even with my 1.4x teleconverter attached. (My Canon equipment was also sharper). See example, notice lack of feather detail and this is the best out of 50:
Untitled by
David Stephens, on Flickr
I decided to test, in good light, using a static subject. I took shots with the bare lens, all from the same distance, and then cropped the bare lens shot and the 1.4x teleconverter shot to match the 2.0x shot. So, the 2.0x is uncropped, the 1.4x is cropped a bit and the bare lens shot is cropped 50%. Here are the results:
Bare Lens
Bare Lens - 3892x2596p to approximate 2.0x teleconverter by
David Stephens, on Flickr
1.4x Teleconverter
1.4x teleconverter - 5481x3656p Crop by
David Stephens, on Flickr
2.0x Teleconverter
2.0x Teleconverter - Uncropped 7952x5304p by
David Stephens, on Flickr
I cropped further so that you can study detail. Keep in mind that I took these all at the same distance, so the DOF shrinks as the focal length goes up. To get the DOF equal, I would have needed to back up from the bare lens to get the perspective and magnification the same. So compare in-focus to in-focus. I want to use real, in the wild, elements, to see realistic detail. BTW, resolution on the camera is 42.4mp. Magnified results:
Bare Lens
Bare Lens - Cropped to 1955x1304p by
David Stephens, on Flickr
1.4x Teleconverter
1.4x teleconverter - 2737x1825p Crop by
David Stephens, on Flickr
2.0x Teleconverter
2.0x Teleconverter - Cropped 3912x2610p by
David Stephens, on Flickr
Sorry for the flaw in my methodology. My focus was on the front of the bunch on top.
As expected, the bare lens is best, even with the severe cropping needed to get the subject the same size. The 1.4x is close and acceptable. I can't accept the 2.0x's performance and will send it back. IQ trumps focal length.