The Barnes loads show the 243 @500 yds @ 1943 fps
The Barnes load for the 30-06 @ 500 yds 1967 fps.

Using the best bullet and load for many cartridges , the 243 does not have much if any velocity advantage. The 243 also lacks the frontal area and bullet weight to compare with larger capacity cartridges. There is nothing magic about any cartridge. Powder capacity, bullet frontal area , bullet weight, velocity and ME combine to do the job. Velocity is but one side of a five sided fomula. The 45-70 which certainly lacks high velocity will do the job just as will a 270 with both velocity and fair bullet weight. 30 caliber bullets of 150 grains and up expand well at 308-30-06- 300 Win Mag velocities. The 180 and 200 grain bullets expanded well at 30-40 krag velocities. Velocity helps bullets expand but most good hunting bullets will have plenty of expansion at velocities of 2500 fps or more. I have seen it for fifty years plus. I have no intention of making a 15 page debate on this subject again. It is MY OPINION that the 243 is on the light side for any big game. Small whitetail and pronhorn are an exception and I have used it with marginal success on these critters. For big game I will always lean toward the 25 caliber and up. I have used them all a lot and thats been my findings. Good Hunting.