I think it's probably very challenging to make a bullet of that size in a J4 jacket. My guess is that when Berger comes out with the 338 it will be a modified jacket along the lines of their thick target bullets. Just a guess, I could be wrong.
I have been wondering this same question myself. However I came across boretech bullets they make bullets for the 338's and the 50 cal and that is it. In the 338 they make a 245 grain bullet with a bc of .869. This is pretty amazing. So I sent them an email about these bullets when I was going to build a 338rum. Boretech told me that the bullets worked great on big game at extended ranges with match grade percision.