I'm not exactly sure what your 30-378 is or isn't doing that's causing you grief but the 338-378 I had didn't like the upper to higher end of the powder range/s for whichever bullet I was using. If I tried to push it very hard the groups were allll over the paper. I personally think those little #3 contour barrels they use are the problem. I think that is way to light weight of barrel for those cannons with the case capacity of the 378 parent case. I think if they came with a Sendero size contour the better performance would out weigh the extra weight.
You could have it bedded, at the least, it wouldn't hurt anything and I think just about, if not, all factory rifles would benefit from a good bedding job.
Maybe have the crown looked at.
If you like it and want to keep it, send it to one of the many excellent Gunsmiths on this forum and have them put atleast a #5, preferably bigger, contour barrel on it with a good match chamber and it will most likely turn it into a much better and consistent shooting rifle.
JE Custom on this forum is a good Weatherby guy. I would PM/contact him and see if he could help sort out your problems.