Check out Christopher Dodd's Recent Blog posts about his experience with the Sony a9 full-frame body and the Sony FE G Master 100-400mm f/4.5-5.6 OSS lens and the Sony FE 1.4x teleconverter.

I was with Chris when he captured some of those puffin shots and I've since bought my own a9/G Master 100-400/1.4x teleconverter. The autofocus is outrageous, IME, but I'll let you consider Chris' words, since he's had his rig longer and he's had the opportunity to shoot much tougher subjects than me. (50-mph puffins). My useage, so far, has been limited to slow, big waterbirds and deer jumping fences. I couldn't be happier. My keeper rate is way, way up versus my Canon rig.

See Chris' blog at:
http://naturephotographyblog.squarespace.com/