You raise a good point about the barrel length. I have owned a made-in-1977, a made-in-1957 and a made-in-1955 Win 94 in 30-30 that all had 20" barrel with sights on the barrel. All also eject straight up, so I did try to install a scope on the 1977 - from the mounts available at the time, I ended up with the scope significantly off-set to the left of the barrel centre line, to allow it to eject and clear that scope tube. Was not really my "cup of tea", although another fellow thought it was just dandy, and he bought it from me.
Looking at some ballistic tables - if one is truly getting 2,200 or 2,400 fps from the muzzle, then 200 yard impact speed is circa 1,500 or 1,600 fps with 150 or 170 grain 30-30 bullets - I think that is likely "border line" for proper bullet upset on impact? Is no doubt that a 6" longer rifle barrel likely gets (some) more velocity over the 20" carbine barrels.
But is likely as much a "sighting" thing. I do not know many folks that can hit an 8" pie plate, at 100 yards, 3 times in a row, with barrel mounted iron sights, from a standing off-hand shooting position. I suspect it is easier to do with that 6" longer barrel, but I am not sure of that. I just measured on the last Model 94 that is here - the original barrel mounted sights might be 17" apart on that 20" barrel - I do have that rear sight removed and installed a Williams aperture sight on the very rear of the receiver, so the distance for that is now 23" - that gained 6" over factory, which I presume is the distance gain with the longer barrelled versions - is probable that the "old timers" could really take advantage of the increased precision possible with the barrel mounted iron sights to be further apart.
So, those older 30-30 rifles likely hit just a bit harder, and allow the shooter to aim a bit more precisely, compared to the "more common" modern carbines.