Well, you can compare ballistics all day long, but that leaves out "the rest of the story"
The Marlin Express series are not only a new chamber, they made additional changes to the rifle geared towards accuracy.
Tomray could fill in the details, but basically the way the barrel is attched is different, and the mag tube is supposed to be attached differently too.
The following quote is from the Guns & Ammo article where they quote the Marlin R&D people who developed the rifle:\
"The big difference here is that we have been used to thinking in terms of 100- to 200-yard shooting with 'hunting type' accuracy," says Tony Aeschliman, public relations and marketing manager for Marlin Firearms. "Now we have a lever action that is a 300- to 400-yard gun, and everything tightens.
"Everything changes as far as how we do final assembly and what the tolerances are where the barrel meets the receiver. For example, if the barrel is not fitting exactly square to the receiver in a 150-yard gun, that doesn't really show. But you add another 150 yards to that and it becomes a big factor.
"The reason, of course, is that everything becomes magnified the farther away from the muzzle the bullet travels. A two-inch group at 100 yards becomes a four- to five-inch group at 200 yards and loses any semblance of hunting accuracy by the time you get to 300 yards.
"So it required us to look at things we never had to look at before because they never showed up before when shooting at traditional lever-gun ranges," Aeschliman says.
"For example, with the attachment of the magazine tube to the barrel, just the tiniest variation can have major effects on the performance of the gun. If the screw that holds that magazine tube to the barrel is over-tightened, that can affect accuracy. If that screw is tightened too hard and the barrel starts to warm up and swell, the torque makes it move and it can act as a fulcrum. So we've been tweaking that attachment a little bit so there's not a lot of stress at that point It's not exactly free-floating, but it's an attempt to keep as little tension as possible on the bottom of the barrel," he says.
In a way, it's like building in flexibility in an aircraft so that there is some "give" to the wings and fuselage to keep them from breaking apart in turbulence. As a result, Marlin has created a "stopping point" on the magazine tube screw so that it can't be tightened beyond a certain point and isn't pressing against the barrel.
In addition, the fore-end cap has been reshaped so it does not come in contact with the barrel, and the surface finish of the locking lug is being given a higher degree of finish to ensure easier chambering and extraction due to the higher pressures of the .308 Marlin Express.
"These are just some of the many little things that we never had to face before," Aeschliman says, "because we never had a true long-range, flat-shooting lever action before. After all, this lever action is, for all practical purposes, a .308 Winchester in terms of ballistic performance. For people who like to shoot lever actions, it's a huge technological step forward."