How do you know that the pressure in your boosted flanged reload is anywhere near the same as the pressure in a factory belted cartridge?
Remember that the 375 flanged magnum pressure is already right up there as far as double rifles go.
The 450 No2 runs at about 13 tons but the 375 flanged mag runs at about 18 tons!! Almost a 40% increase.

Kynoch currently lists 375 flanged magnum ammo along with many others including 450 No2.

Kreighoff doubles in cals up to 375 have a moveable wedge and regulation can be adjusted.
Don't know how well this system works though.