Some time ago I promised to report on the Sony 1.4X TC when used with the 70-400 mm G lens. The first few shots were very poor and I sent the converter back to Sony, who simply - and quickly - replaced it, no questions asked.
As you can see with this shot, it does just fine, certainly for wildlife where soft corners are of no concern. This was shot at ISO 320 and f16 - hand-held at 1/125th! Not bad for an effective 560 mm lens. The image did require more than C1's default sharpening, however, and actually could use a bit more. But a promising start!