seeing that the 30-378 is already way overbore , I don't think that you would be gaining much by making a 25-378 unless you were using a 40" barrel and some sort of super slow powder.
From what I've seen , I think that with standard barrel lenght say 28" , the 25-300Win mag will do about everthing that the larger cased rounds will do with less powder.
Speed is a great thing in some cases but most of the time your gonna find that a round will shoot its best at a lower speed tan you want to get , like the 6.5-284 it could possibaly run a 140gr bullet out at close to 3000fps with a long tube and slow powder and around 60 or grains of powder. and a 6.5-300RUM might be able to run that same bullet out at maybe 3500 fps but I'd be willing to bet that it would not be nearly as accurat as the 6.5-284.
And I don't care what you shooting ,you just can't miss a critter fast enough to kill it !!!
