Generally speaking my question is if we can increase the rigidity of a tube or any structure made of non-expanding material (like a compressed gas cylinder) by filling it with compressed air, say at a pressure of 4 MPa (so 40 bars) you suggest, or more. If yes @dougselsam would be happy for the shaft of SuperTurbine ™.
Tubes and valves. Far above 40 bars.
I agree with @aokholm 's comment: I don’t see the use of the piston which adds complexity and risks that @Rodread explains when using a thin shaft.