We have played with the 300 Norma imp and necked it up and down. It will produce about rum performance with about 10g less powder and a shorter oal. The Lapua imp will produce about 200 fsp over the rum depending on the reamer. Yet still a bit shorter oal than the rum. The Lapua imp is pretty tough to beat until you go to a Cheytac parent case. Whole diff world.
I like the RUM's over the Lapua/Norma stuff just because I don't keep a rifle the same for long so it gives more options when rebarrelling. The 7mm Lapua Imp (AM) is about the best N/A hunting cartridge ever built though.