Ally cage.... ha ha ha.
as for stainless it may be strong under certain loadings but is also brittel and less mallable than mild steel.
If the car was hit hard the cage may snap rather than just deform.
I have seen a stainless Fusion Fab cage and it must have been a show only cage because it offered no protecton what so ever.
It was mounted at the back off two pieces of 2mm sheet stainless in single shear.
If it offers no protection and it adds a shed load of weight, what the hell is the point of having it?????