When you are attaching TC to your lens you are adding another flange tolerance to your setup. If it is slightly off you will have less sharpness. That is why some people report great results with TCs and some don't. When lens is made all elements are adjusted together. TC adds another variable to the setup. You also have to micro focus adjust the lens with TC. TC acts as a crop device. It takes the small area of the center of the image and makes it full frame again.
Now, at F14 your physical aperture is actually at F8. At F16 you are above F8 and that is where some people start seeing diffraction.