Extreme range is over 1000 yards as defined in the first post by Len stuck at the top of the forum.
I've shot a LOT of elk between 600-900 yards with a 6.5, all with a 6.5 SS shooting 140 gr + bullets around 3200 fps, elevation all 6500+ ft.
Typical impact velocity at 800 yard at my lowest elevation is 2200 fps with 1500 ftlbs of energy, energy is just for the guys who want to know as it means zippo to killing at long range!!
My by far favorite and best performing bullets are the 140 Berger, 150 Matrix and 160 Matrix with most kills with the 140 Berger, I've tried most all of them and these are the bullets that maximize the 6.5 lethal range from what I've seen inside dead elk.
I've shot one elk twice in these ranges and that was because I was testing a "hunting" bullet and the wound channels suck compared to a bullet that is better suited. All other elk died within feet of the hit regardless of hit behind the shoulder or through the shoulder, I have never had the Berger 140 fail to penetrate an elk shoulder, one of the few below 200 gr 30 cal I have seen this.
I will shoot an elk farther than I will a deer, elk open bullets better than deer do and since that is what kills at long range I've found it works well, elk will also stand there wondering what hit them too allowing follow up shots as well better than a lot of deer.
One thing guys have to realize is your using a 6.5 to have a low recoiling, precision shooting rifle and the best ballistics that is very shootable with barrel life, inside that criteria, you better have your stuff wired TIGHT beyond 800 yards, beyond 1000 is a very, very narrow window even at my altitude. Targets at a 1000+ are not a challenge but shooting game is a whole different ball game.
I'm only capable of shooting game at 800+ with my 6.5 SS if I'm shooting it a lot and I'm on top of my game, muscle memory is established with the rifle and the wind drift is almost instinctive, confidence in shot placement is within a couple inches, I'm not talking hitting the front half of an elk I'm talking targeting the aorta just above the heart or neck, broadside or quartering away shots only. On paper at a 1000 yards when I'm on this gun a lot and I'm wired it's consistent 3.5 inch gun and will shoot smaller when I'm watching my tracking for three shots, never shot more than three.
This is not to be argumentative just to help those who have little to no actual data for this kind of hunting or shot with this cal and with the ballistics provided should give a guy and idea of what's realistic.