I am looking to increasing my focal length for the best pictures possible with a minimal weight penalty to my already 15 pound traveling photo kit. The store salesmen told me that I have to expect a significant quality and focus speed hit if I was to put a TC-20E III between my D700 and my 70-200 2.8G VRII and claim having received many bad comments. According to him, the TC-17E II would inject almost no or negligible loss of performance and the difference between 340mm to 400mm should not be an issue for me.
I am not sure if his comment relates as much to quality issues or to the fact that the TC-20E III is almost impossible to get while he has plenty of TC-17E II at hand. He was trying to push the Nikon 80-400 F4.5-5.6 or the Sigma 120-400 in my head but I am afraid my back would not agree.
I am more into shooting static things, not so-fast moving wildlife , architectural details but I surely would not mind acquiring the capability to shoot birds in mid-air.