I know that the question was about an 30-378whby mag. I have never owned or shot one however I do own an .300whby mag. And my neighbor owns a .300win mag. He has the advantage of cheaper brass and less powder used in a case so his ammo is cheaper than mine. However I can load mild to medium loads of 165-210gr. Bullets and I have a definite advantage of velocity over his rounds loaded medium to hot. And we know that F=M×A so I there by have energy over his .300win mag. I also will have enough case to run the 210gr. Projectiles at 3085fps from an 26" barrel loaded out with 82.5gr of H1000 which is stated to not go below trans sonic till 1800ish yards so a good mile which is well beyond an ethical kill shot in most large game. (In my opinion) I also had a friend from Idaho whi swore by his .300 RUM after we had a talk about the loads and how much powder difference there was between the two for almost the same result he bought the .300whby as to why he never stated. I think that Randy aka The Real Gunsmith has done a superb job In explaining the whole dilemma of the .30cal mags on his video. 300 PRC on YouTube. I believe that an old tried and trued method is best over a new one that is until it has been truly tested and proven. In my limited experience I believe that a more accurate way to compare the .30 mags would not be by bullet diameter but by case capacity alone then we would see who is on the top.
To try to answer the question.
I don't know if it's better to head space off the shoulder or a belt.
I also dont know if the shoulder angle of a PRC, Rem, Win or Whby is superior.
I wish we could take the same case with all the same components and only change shoulder angle and neck length both independently and test the results to have a definite answer to this question. However I think that it is likely the old 30-30 mentioned above that some like one cartridge over the other why??? Because I killed more deer with my .300whby than any other rifle of mine with exception of one. The .22 win mag it's my favorite for the job up close. But there we go again with the whole ethics and legality bit.
Truth is what ever you like is best. And right now ain't a time for change with hard to get bullets and components unless you just like to clean and look at your ri