If you have a .243 shoot it and see how it does for yourself. If you are planning on one to shoot silhouette I would not bother.
If you do shoot a .243 I would try to acquire some of Tubb's 115gr bullets. Rams only make up ten of the animals but it is frustrating to hit an animal and watch it stand.
I asked a similar question awhile back when Tubb's 115gr bullet first came out. Search the archives to see what was said about a 6mm.