a good friend of mine has a deer rifle built in this calibre, and it is stupidly accurate. its built on a BAT action with a 26" very heavy barrel.
all things being equal, the 6xc will outperform the .243 in many ways, such as better accuracy, less recoil, less muzzle flip, less heat means more shots fired before accuracy drops off, longer barrel life to name but a few.
fire forming is not required and he has not experienced any feed problems, it feeds perfectly from AICS magazines, factory brass is available but being an "accurate" round, many reamers are tight neck and brass requires neck turning before you can use it, and obviously carefull brass prep and reloading are required to get best results.
so really, you need a big heavy, well fitting rifle to take advantage of the accuracy potential of the round, if its just a sporter weight rifle, you are probably not going to see any benefit over a .243 if smacking deer in the ribs is all you want it for.