Actually, I don't think the plastic would fail. What concerned me was the flexing of the bolts that would be allowed by the plastic, posiibly causing the bolts to fail. A1s only have 4 bolts holding the stubs. Some A2s probably went to 6 bolts for a reason.
If you go with the plastic, make sure you upgrade your bolts to at least 12.9s... but those cost money too (I think Shine includes longer extra strength bolts in his kits).
Having melted plastic wheel centering rings, I just don't like the stuff anywhere near a brake; but admittedly, the rears should never get as hot as the fronts.