Norma WBY brass costs just over $1.50 per case. Not sure how accurate it is but I read LRM brass is $1.25 per case and probably as good or better than the Norma and it's annealed. I ordered my Hornady 300 Dakota brass directly from Dakota and a lot of 100 was about $2.00 per case and of very good quality. Dakota usually has inventory in stock.
If you merely necked down the LRM or 7 Dakota
, fire forming would be next to nil and could be accomplished with load development. If you improve the WBY case, there will be some work and cost involved, so the cost f comparable quality brass would close to equal Dakota brass cost.
My next 6.5 will be a 6.5/300 Dakota. If I was looking for your wanted performance, it would be the LRM or 7 Dakota. I can not think of one good reason the WBY cartridge would be better than either of these.
Interestingly, you also almost never hear of belted mag cartridges being improved.