The longer cases will have to have a higher wad column - or more of a slower powder.
I would actually think that a 2 3/4" case would hold all I wanted to shoot. I've read that Magtec brass sits normal dies, whereas the RMC doesn't. There was an article on loading brass and plastic in either the DG Journal or Handloader magazine - I think the magazine rather than the journal - recently- ie: last couple months. The author used 12 bore only, IIRC, and obtained some pressy impressive ballistics from the 1.4" magtec brass. I have some in 16 bore, takes large pistol primers and works beautifully - annealed first.