In theory may be, and that too if you are going to a take just a few shots and call it a day.
In practice a sports photographer normally shoots 1000's of images per game (i.e. within a scope of 2 hours), and for all those shots to be squinting at the 1.5 crop box and also keeping the same eye for possible action from either end outside that 1.5 crop but within the viewfinder, and also keeping your other eye out for possible action elsewhere, or some dangerous projectile flying out at you, or you stepping on someone/thing else, or bumping into the referee, etc... Believe me I tried it just for kicks, it’s not worth the trouble, and neither required for a seasoned photographer. And as stated in my previous post, I simply do not see any merit in shooting dx on an fx format.
Yup, after a few thousand shots one does catch on...sports is about anticipating the next place where the action will be, getting one's position, and hoping you are not going to be squashed by some participant in the action. Having the image full size in the viewfinder helps along with one eye open to the real world.
One idea I will be trying with the 400mm lens is to have an external set of "sights" on top of the lens/body and use this to capture the place I want the image, but be able to see all the action around the camera. This eliminates the DX on FX advantage as far as seeing the area around the actual image.