I think the Retumbo load might be your Huckleberry. The Mashburn is an interesting wildcat, which version is it since in my reading there seems to exist 3 variations one being a resized .300 Win mag case, and 2 others involving either necking down or necking down and blowing out a .300 H&H case? I'm curious if the Nosler Ballistic tips would give you the same POI, I found the 140's to be interchangeable and just save the Acubonds for hunting.
Of course the 150 NBT has served fairly well for all around hunting in my own and friends large cased 7mm's. It would give me a little more confidence if I had to put one into the shoulder of a moose or elk if the bullet was bonded.