Probably just needed more shutter. The pixel density requires alot of shutter and I noticed that normally on 5D3 and say 300mm I could knock off a 1/320 shot and be sharp as a tack but it required 1/640 on the 7D2.
Don't think so. I was shooting birds (not in flight though), and kept the shutter speed in a range of 1/1000 - 1/1250. Beautiful shots without the TC. Not so with it. I am not saying the IQ was bad - it was acceptable, but cannot compare with the 5D4.