Re: primer changes
I changed a few loads from fed 215 gm's to cci 250 M's for the same reason. Just could not get them. It all worked out good for me with no noticable difference. I would think the biggerst concern would be,... it could affect the ES of the load. So you may want to chrony a string of the new load to make sure it is with in your tolerance.
So I guess the simple answer would be, both 215 gm's and cci 250's should work fine in your 30-378 with retumbo. If you are close to max with Retumbo I would back off a couple grains for the first test.
Hope this answers the question.