I think with Big Doubles the maybe the weight requirements for recoil can allow older guns to "get by" with less stronger steels than smaller calibers.The distances and game involved do not benefit the shooter by lighter rifles and faster velocities.

How about a 9.0-9.5 pound scoped 450/400 with "modern double" with stronger lighter steels, stronger action, good balance between the hands,shooting a higher velocity 400 gr bullet for non dangerous game at longer ranges?? I shoot a 416 rigby rifle about that weight at a much faster velocity.
Not needed?? For longer distance shots(out to a maximum of 300yds) a 100-150 fps increase in velocity flattens the trajectory by 5" and increases the energy delivered by almost 20% at maximum distance.Thus we would have a scoped Double rifle that would be more flexible at distance.

Pressure concerns?? I don't know.Just have to evaluate and see if a modern Double action can safely tolerate higher pressures.This may be a pipe dream but it is intriguing--