You would give up nothing to Lapua by shooting ADG or Peterson brass in 28 Nosler.The short answer is the longer neck and throat as well as Lapua brass.
Regarding the 7 PRC, I'm thinking if I want mid level the 7 SAUM sings to me as a short action targeting the 175 ELD-X or similar Berger bullet. I do think it's hard to go wrong with any of these. I was pleased with my 7 Remington mag and regret selling it, but if I'm going to build one 7mm from the ground up I want it to be just exactly what I want. Just still having fun trying to define what that is.
If you're gonna build custom anyway, throat and twist the Nosler properly and enjoy. No need IMO to reinvent the wheel with an obscure wildcat that will require custom dies and such.
Lapua brass is good but not the end all, be all.