Recently I stumbled upon this write up from 2 years ago:http://www.thewanderinglensman.com/2015/03/is-digital-tele-converter-feature-in.html
When I travel I always shoot raw, but in the last couple of years I have started to shoot jpeg quite a lot again when I am not traveling and just doing my daily wanders. Of course, we all know that these digital teleconverters just take a central crop and then interpolate it up to full size, but from playing around a bit with the one in my 20mp Olympus PEN-F and using the 75-300mm at 300mm it does pretty well. As a reminder, 300mm is 600mm in FF terms so with the 2x digital teleconverter that is 1200mm in FF terms!
Jeez, Louise!!! Pretty cool!
In order to do a 2x conversion it needs to take the central 1/4 of the image (5mp) and interpolate that to 20mp.
5mp: 2592x1944 (like the 5mp Minolta D7i I used to have)
The resulting 20mp jpeg looks pretty good, but on your computer if you reduce it back to 5mp it looks really good. Actually, just reducing it to 10mp or 12mp (4000x3000) also looks very good.
Shooting without using the digital teleconverter and then cropping on the computer (especially if using raw) and then resizing to 10mp, 12mp, or 20mp is similar, of course. Some in a discussion on another forum sometime back think that, at least Olympus, does a better job in camera, but I don't know. A really big advantage to using the digital teleconverter though is that the EVF shows what you will get at the proper size rather than trying to guess the central 1/4 of the image. You can see details better, see that things are in focus properly, etc.