1) It probably won't fit right.
2) It'll probably lower the rigidity of the car...the way it's structured.
3) That thing is going to be prone to cracking, and allowing the glass to follow suit.
4) It's not really CF. It's fiberglass...covered with CF.
5) It'll probably be less safe in the event of a rear end collision.

Sounds like it's worh it to me...