I have replaced those on two cars. They weren't damaged as bad as yours, just minor cracks. From my observation, these are brittle and will break away in most cases before damaging the frame of the car. In your case, I would be sure to check the black brace that crosses between the engine and transmission and the diagonal braces on the bottom.
You may need the assistance of a ratchet strap to get everything aligned correctly for re-installation. This would be a great time to replace you engine and transmission mounts while you are in there.
Some posts that I remember:
http://986forum.com/forums/boxster-general-discussions/63145-aluminum-diagonal-brace.html
http://986forum.com/forums/diy-project-guides/63154-replacing-rear-suspension-bracket.html#post510170