Don't understand the OP position on mirror less bodies they have there afvantages for birding and motor sports, I would suggest renting and trying, your biggest issue will be the learning curve, setting up the camera and memorizing the custom buttons. Then adjusting to the particulars of the viewfinder and sequence firing. I've used the Oly and Fuji two body generations withe each no I have a X Pro 2 kit we'll sort of the 16-55 and the 100-400 w/ 1.4 ex I don't know how much faster you would want the camera to focus but if set up properly is VERY FAST. There is a review on Fuji site by one of the foremost Motorsport photographers using the 100-400 at the track you should watch it!
Regards