If we follow the "school o thought" of 1000 ft-lbs of energy and at least 1900'/sec bullet velocity on target.........the 243 with 105 Bergers or 105 A-Max's (3000'/sec mv) has the remaining energy and velocity to do the job at over 600 yds, even 700 yds at the altitudes I hunt at.
Calm conditions, steady rest, good trigger technique, plenty of time, and a rifle capable of 1/2 minute accuracy (every bullet hits within 2 or 3 inches of PoA) .......I personally wouldn't have a problem attempting the kill with a broadside shot presentation at 700 yds.
I am talking antelope and deer, just my opinion; but I think attempting such a shot on elk with a 243 is just asking for trouble.