Here is a Canon EOS R7 vs EOS R8 comparison review, and a highlight of…
Canon EF 200-400mm f/4L IS 1.4x Sample Pics (ephotozine)

ephotozine posted a set of sample pics shot with the Canon EF 200-400mm f/4L IS 1.4x lens (Amazon | Adorama | B&H) and a Canon EOS-1D X. Regarding the auto-focus they write:
Focus was extremely fast when used in combination with the Canon EOS 1D-X, with AF tracking enabled to keep track of the high speed planes at the airfield.
ephotozine’s post comes with more samples, and a set of pictures of the lens.

Lens specs:
- High-Performance Telephoto Zoom Lens
- Canon EF Mount
- Aperture Range f/4-32
- Built-in 1.4x Extender
- Three-Mode Optical Image Stabilization
- Ring USM Autofocus & Internal Focusing System
- Power Focus Mode for Smooth Focus Shift
- One Fluorite and Four UD Lens Elements
- Fluorine Front & Rear Lens Coating
- Highly Resistant to Dust and Water
Canon EF 200-400mm f/4L IS Extender 1.4x
Amazon | Adorama | B&H
rent it at Borrow Lenses