G'day Nitro
If you had a double in 375 flanged and loaded it up to 375 belted velocity it would be very unlikely to regulate.
If a rifle maker built a double in 375 flanged and regulated it for 375 belted velocity it would be unlikely to regulate with factory flanged ammo.
If you owned a rifle proofed for the flanged load it would NOT be wise to increase the velocity and pressure above that proof.
Personally I don't like the idea of high pressure cartridges being used in doubles regardless of modern action designs or so called better steels being used.

As far as choosing between 375 flanged and 9.3/74, I'd be quite happy with either.