243 is 0.100 longer case, don't seem like much but: has a short neck and short freebore for lighter bullets. Want to increase efficiency, sure! Add another 0.120 freebore to make room for heavies. Now you're running 2.9"+ to 3" coal. You'll be jumping bullets a long ways(not a bad thing) but having mag length constraint with the longer high bc stuff like atips, 109 berger. All for what, 100fps advantage? Not many guys in the know run their rifles in the top velocity node, most competitors shooting 6cm are 2950-3050 with long barrels, some even run below 2900. The extra velocity from 3000 fps to 3100fps is of little advantage. Yeah you might have 0.1 mil less wind at 700y, but can you shoot the difference on plate or paper? I know I can't. 6cm is the easy button, tons of good factory ammo, easy to load for, great brass from lapua or alpha, my advice go 6cm, it's a no brainer for me.
The 243 is an excellent varmint cartridge, and 100gr bullets ar 3200fps are great deer/Antelope rounds if the bullet construction is good. The 243 has its place! Lot of guys switched to 243ai, and get better barrel life with the improved shoulder and slower powders VS standard 243 running same bullets at same velocities. These were longer throats for match bullets.