Gadget Man
The 1.4X teleconverter works quite well and I use it together with my FE 100-400MM zoom lens. On the 400 it increases my reach to 560MM with just ~1 stop of light loss. I dont see any other issues such as reduction of sharpness etc. it seems the only drawback is a bit less light. I did notice one drawback, not all of the features of the camera are totally compatible with the 1.4X tele. For example you can use focus peaking in manual focus but the in focus zoom mode does not function so its a little harder to get the shot sometimes. All in all this is the perfect companion for the FE 100-400MM lens for your sony. BTW my camera is the Sony A7R3.