Just saw this, so I thought I would respond in case you are still looking for information.
I have a 338 Norma Improved. I shoot the 300 Bergers out of a CIP length AICS magazine. Neck shoulder junction is above the donut, but with Norma brass the primer pockets will be gone before you ever see a donut in my experience (although admittedly I load pretty hot because that is where I see the best precision with this cartridge). Lapua brass should offer much stronger case heads though.
My chamber is Robert Gradous' 338 Warthog. It has a 40 degree shoulder and is pretty much a max capacity 338 Norma Improved. I don't have specifics on freebore. I had custom Whidden dies made from the reamer print and they work great. Using a case full of RL33, velocity with the 300 Bergers is an honest 2900 with a 28" barrel, Magnetospeed and drop verified. That load shows minor pressure signs. This cartridge definitely likes to be loaded hot.
Was it worth it to go improved? In my opinion yes. That rifle is literally one of the most accurate rifles I own, up there with 6.5x47 and 223. Well under 0.5 MOA. I have multiple examples of three shot groups in the zeros, 5 shot groups in the 0.2s. I have never once trimmed a piece of brass. And you simply can't argue with a Berger 300 going 2900fps. That is a hell of a lot of performance. Also Robert Gradous is an absolute master of his craft and I can't recommend him highly enough.
