I don't know that alot is gained by improving the 300RUM but I do know the Lapua case picks up quite a bit when improved so just based on case capacity a 30-338Lap. Imp. would hold a pretty sizable advantage over a 300RUM Imp.
Kirby Allen W/APS Rifles probably has one of, if not, the more popular 300 Lap Imp chamberings that he calls his 300AX (Allen Express). I'm not exactly sure what Kirby's 300AX has for a case capacity but I plan on finding out in the next few months because he is building me one. Big 30's shooting heavies is still my favorite.
I know you didn't mention the 30-338Lap Imp as one of your choices but if you are considering improving the 300RUM why not consider improving the Lapua case as well.