My 20" Winchester makes an easy 2,300fps with 220gr. I have driven 270's to 1,960fps with good accuracy and without excessive pressure - by measurements - normal for all factory dup. loads from manuals - one must use the correct powder is all.

With a 24" barrel, that ammo would probalby exceed 2,000fps by a tich.
More speed from this round is not necessary. In both 1981 and 82 I shot a moose with my wife's .375 Big Bore, '94. The load was a soft pressure 42gr. H335, CCI250 primer with a 220gr Hornady for a sedate 1,940fps. The cow took 1 1/2 steps and fell dead, the other, the bull, ran 4 steps and fell dead. Both were lung shot although the cow's bullet went through one shoulder as well, to stop in the other, however, 1 shot each was suffice.

Is the .375 Win. as good as the .375 2 1/2"? - don't care - seems it will match the velocity of the factory 2 1/2" , but there are no 270gr. bullets from North American suppliers suitable for these low speeds.

Whether the .375 Win. makes double the 2 1/2" pressures - another moot point - doesn't matter. With my loads, both hunting and maximun, the brass seems indestructable and is long lived - haven't lost one yet - been loading and shooting it since 1978 - probably better lived than the brass is for the 2 1/2" - don't know, doesn't matter. I use 30/30 brass blown out to .375" for my hunting loads - works perfectly at the moderate pressures I normally shoot and with so much once-fired .30/30 brass at the range here, it's an endless supply. When hunting, I just let them fly, no need to pick them up - they're free.

9.3x57 - I like your idea of the Marlin chambered for the .38/56 IMP. The capacity would be over 70gr. and exceed the .9.3x62 and approach the .375/06IMP, both at 78gr. capacity. I mean it would exceed the 9.3x62 factory 286gr. ammo, but be short of modern-type handloads in either of these '06 based rounds. Yes, I know the 9.3x62 is not an '06 based round, BUt - brass can be safely made for either round using .30/06 brass.