Alan,
I'm not trying to be a wise guy at all but am speaking from personal experience. I love my 6.5 Sherman and have killed in excess of 25 bulls with a 6.5. Last year I lost a bull that was shot in the chest at 1010 yards with a 140 leaving the muzzle at 3150'. I feel quite confident that it was a pretty good hit but will never know for sure because the elk ran off after lying there for three hours. The problem with the 6.5 is not much frontal area to start with and for a non expanded bullet, (pinhole)! You can get away with that with a .338 but not a 6.5. The approx. velocity of a 140 Berger (for instance) starting out at 2800' would be 1700' at 900 yards. There is no way that this bullet will will expand CONSISTENTLY at that velocity! I may get some argument on that, but I have done a lot of testing and that has been my experience. If I WERE going to shoot long distance, I would use the A-Max but then you have a terribly explosive bullet at closer range. The Berger is good IMO in a 6.5, above 2100-2200'. I have no doubt that you could kill a lot of elk at 900 yards with the scenario that you gave but the odds of losing one would also be quite high. I hesitate to recommend what the max yardage is, but if you choose to use my logic, I think you can come up with a good answer
Good luck and good hunting......Rich