Basically, you will have the F2.8 that either 540 or 640 have for sports. And the reach of 560mm F4 which is almost as good as 640. You would have a smaller physical size than 640.
Put on a TC-20 E III and you got yourself a 800mm F5.6.
When I saved enough, I am getting a 428. If you shoot Canon, the price is much more reasonable than Nikon. A used Canon EF 400mm f/2.8 L IS USM is in the $6500 range while Nikon ones are above $8000