It depends on the bullets and loads you choose, as always.
So here we have a 143gr 6.5CM against a 30-06 178gr. Fair comparison? I don't know, it's just one I grabbed online - personally I shoot a 165gr 30-06 which is even closer. But either way, there's almost no perceptible difference in trajectory out through 600+ yards, way more than most people shoot.
Now, at 500 yards, that 6.5CM bullet is delivering 1306 ft. lbs. and the 30-06 is delivering 1567... which is "more"... but since pistol hunters are allowed to use loads that deliver "at least 550 ft. lbs." and archery hunters take elk with arrows delivering 60 ft. lbs., I'd argue that's still well more than enough - and that at the 200-300 yards most hunters usually shoot within, both rounds are "well more than sufficient". As said above, shot placement is everything.
Calculates the ballistic trajectory of a bullet fired from a rifle, handgun or other firearm. Produces a ballistic trajectory chart and table that shows the drop, velocity, kinetic energy, windage, and trajectory of a bullet.
www.shooterscalculator.com
You can monkey with the numbers any way you want, but I still don't see this as being an "underpowered" round. If the shot is within your ethical limits/capabilities (in terms of being able to accurately place a killing shot at the given range and animal position), both rounds should take that animal easily.