I always hear comments like "I don't use my 1.4x and 2.0 converters because they make my images soft". That is true that both the converters will make a photo "softer" if you compare it to an image not shot with a converter, BUT the image does not become unusable!! Two factors that will influence the sharpness, one is technique and the second is the sweet spot of a lens.
Technique of Coverter Shooting
When you add converters you are increasing your focal length, it sounds very obvious but most people forget about the impact of that. Longer focal lengths increase the chance of camera shake. Remember the rule to prevent camera shake, shutter speed should be equal to the inverse of your focal length. What does that mean; well if you are photographing with a 400mm lens and you add a 1.4x converter on then your focal length changes to 560mm. The inverse of 560 is 1/560 so you need a shutter speed of 1/750 sec and faster to prevent camera shake, and even faster if you add a 2.0x converter on. With a 2.0x converter your focal length becomes 800mm so you will need a shutter speed of 1/1000 sec or faster to prevent camera shake. Yes, IS (Image Stabilizer) does help and you can get away with slower shutter speeds. All I am saying here is check your shutter speed to make sure that you are preventing camera shake!!
Sweet spot of a lens. Lenses are usually designed to give the best resolution at 2 stops from maximum aperture. This means if the lens has a maximum aperture of f4, it will perform best at f8. This rule applies with using converters as well, so when you use a lens with a maximum aperture of f4 and you add a 1.4x converter the maximum aperture becomes f5.6 and therefore the lens will perform best at f11.
To summarize, if you use a 400 f4 lens and add a 1.4x converter then the lens becomes a 560mm f5.6 lens. To capture a sharp image you would like to photograph at 1/750 sec at f8 or f11.