Assume he throttled back the load to try and replicate fps at 300 yards.

We've done the same, years ago, with various X bullets (yes X) and found they expanded in media (as noted above) down to 1700 fps as long as bone was hit.

Some may disagree, but IMO the best use of Barnes is to shoot through both shoulders and break the animal down.
Very little meat is lost and tracking becomes a non event.