That may be true, but I don't consider stopping learning the badge of honour some do I suppose.
I base my previous statement on a little knowledge of fluid dynamics and more specifically hydrodynamics in the way high speed projectiles work and shed energy when encountering large liquid filled sacks of muscle and bone.
Some people like to use KE, and never bothered going any deeper than that. It's a measurement of some sort, but after a little thought it's not a very good one concerning large game. If it was a perfect measure, the Taylor KO index wouldn't be used, or any of the other measures. But it sells rifles to neophytes, and marketing types like it. Mostly the same group buys a lot of stainless, in the mistaken idea that it won't rust, plastic because "your zero will shift with wood", and large barrel gaps to get "accuracy" too, but that's a story for another time. Mostly marketing because it sells.
To put it very simply, ignoring the mechanisms of how on game performance works, most hunters have accepted that big bullets are for big game, and putting it to extremes it's very apparent. While it's entirely possible to get a hot .22 centrefire up to sedate 45-70 energy levels we know that one of them will never be a buffalo gun. Use any bullet you want, and it won't get there. Yukon F+W recognizes this and specifies a certain grain bullet and diameter for a minimum when hunting bison. Have seen a very slow .458 at BP velocity drop a zebra so hard it's nose bounced off the dirt with a complete pass though. Good luck achieving that with a .22 CF.
Back to .243 vs. 30-30; Buffalo Bore offers ammunition for each, and specifies suitability of use. Of note that they (Tim presumably) says this about the premium monolithic load for .243: "Due to the controlled expansion characteristics of the TSX bullet, you’ll get penetration deep enough for killing big deer and up to 300 lb. black bear or hogs." And to be fair, this also: "I do not advocate hunting elk sized animals with even the best 243 big game loads, but it could certainly be done." And this was said about the heavy bullet 30-30 BB load: "designed so that the person who owns a 30-30 can now reliably kill elk or moose sized game. The 30-30 can now be carried as a defensive tool in grizzly country". Tim Sundles has killed probably over 60 cape buffalo now, and has a pretty good handle on what works.
Put simply there is 70 percent and getting close to 100 percent more bullet being launched from a 30-30 over a .243 and even with good monometals, the .243 is not penetrating as deep as a good 30-30 load, and lacking the wound width the small 30 cal delivers.