No, I do mean factory action. Alot of the performance you hear about are from rifles like the SAKO, AI and customs. All have nice tight lock ups that allow for warm to toasty loads to be used without serious case expansion issues.
A typical Rem 700 has a bunch of issues from moderate/poor lug contact, misalignment with bolt face/chamber and raceways,etc. All the little things that a smith blueprints out when a factory action becomes a 'custom'.
All they are doing is making the lock up straight and tight. Then there is also the issue of chamber dimension of which I have no idea what is in the Rem.
A combination of high tolerance in the action/chamber coupled with any bolt movement during firing will cause cases to bulge at elevated pressure.
The 'pressure signs' commonly quoted show up in this type of rifle.
So, that will limit how much pressure/how fast you can push any bullet.
The barrel only affect on speed/pressure is in bore size/roughness and length. Hopefully, these barrels will shoot accurately when fired fast.
Otherwise, you may have a rifle that performs no better then a 338 RUM.
As to building a 338 LM, just depends on the bells and whistles but even if it is a bit more dollars, you end up with a purpose built rig that will shoot more accurately, most likely at desired velocities with reduced recoil.
If Savage announced a 338LM bolt head in the near future, building one for what a Factory Rem costs now is very possible. If single shot, even less money.
Jerry