How a bullet performs and where it is placed is far more important to me than how big and fast it flies. So, in my mind, much of the answer to your maximum distance question, depends on how accurate the shooter can be with your little rig and how well constructed the bullet is.
I don't have a 243 but I have a 257 Roberts, which is similar in size and velocity. It's deadly accurate but even with Nosler Partitioned bullets, I set my deer shooting limits to under 400 yards and that would be in perfect conditions. Small lead or copper pills are like feathers when the wind blows, so air turbulence is definitely a limiting factor.