It might have a 3 or 4 grain increase over the 7x375R, because of the .550 base vs .532 with both having about the same taper, the long and short is that you stand to gain virtually nothing except a hell of a lot of extra work for no good reason.
RDM, I did the very cartridge you're asking about back in 1999 before these other brass options were available. The Dakota brass was crap back then but has been upgraded since about 2006. My cartridge case is 2.555" with 35* shoulder and reduced taper. It's called the 7mmRHB and performs very well. 3200fps and 1/3 moa @ 1K with 180 Bergers. You need a set of form dies and be capable of annealing and neck turning to form this case.
As others have pointed out it is a time consuming way to get to where you would be by necking down a 300 Dakota to 7mm. I designed the 284 STAK which was the 300 Dakota necked to 7mm with a 40* shoulder and reduced taper. It's performance is nearly identical to the 7mmRHB but very easy to make.
The 7mm Dakota is a smaller case than the 300 Dakota ( about 7 grs) The improved 284 STAK holds 3 - 4 gr. less powder than the 7mm RHB even though it is the same length. It is smaller in the body by .003 or .004 depending on the lot.
Good luck with your project. PM me if you would like more info. ---- RHB