I am testing the Viltrox 2x on the Sigma 300-600 f4 mounted to an A1ii body.
So far so good. I have only received the tc today so it's early days. I'm using a gimbal for testing and playing with still subjects. I have been using manual with auto ISO and exposures are basically equivalent with no intervention. With manual ISO you need to add two stops.
Focusing is great. Super snappy and subject detect still works well. It's not quite at the same speed as the native lens but very close.
There is definitely more detail with the 2x than there is cropping from 600mm. See an example below with the lens at 600mm on the left at 400% and with the tc on the right at 200%. EXIF is not recorded correctly (which is why I guess it works) so look at the higher ISO for the TC shot. I am also testing what the difference is between 600mm and no TC and zoomed to 300mm with the TC. Here the native lens does pull ahead at the same magnification detail wise. In the below example focus point is slightly different so please take that into account. The shot with the TC also has a bit more DoF.
At distance and in lower light the additional environmental conditions seem to negate any advantages of the TC. So far it looks like at short/mid distances (say up to 20 meters) and smaller subjects. So useful but not game changing.
Images have been downsized due to website file size limitations. Unfortunately, that's hiding the differences I can see.
