Re: 7/300 wsm
I had a 7/300wsm. The benefit is as you already mentioned. Straight 7wsm has a touch more case capacity, 7/300 wsm gives you longer neck.
For brass prep used a special cut 7wsm neck die to do the initial neck down as I used 300 wsm brass. But then used Redding 300wsm Neck and FL Bushing dies with the proper sized bushings to finalize my cases. Only used the 7wsm as it came with the gun and didn't have to do multiple passes changing bushing sizes with my redding set.
Or like already mention use 270 wsm and neck up.
I got some Norma brass, but mainly used Winchester due to loosing them at matches. If you are not worried about loosing them, Norma was more consistent than the Win. brass.
Mine was built on a long acion Remmy 700. So didn't have to deal with seating depth issues. Ran 180 hybrids around 2930-2950 with H1000. The gun really liked the Hybrids.
These were just my experiences and others may find them different.
Greene