I'm certainly not knowledgeable on WW2 fighters however I know that -5G would be beyond their limit load factor.
I also know how an aircraft responds to control inputs and I've been to -5G many times. Response depends on the rate of control application with airspeed also a big factor. Flying along upright and suddenly hit full forward stick, full left rudder with full left aileron lagging a tad results in an outside snap roll. Easy to calculate the G based on entry speed and the stall speed at -1G.
Slower control movements - obviously forward stick into a dive results in a speed increase. WW2 fighters have similar flight load limitations as current GA airplanes. Rolling G limitations and Va, maneuvering speed. Something will break doing that.