Mud flaps may not reduce mpg in any measurable way in a TDI, but it could on a hybrid since they have much less torque/hp. I notice that with the a/c on or off, it doesn't make any difference in my mpg's, but in my previous hybrid's it did due to the higher electrical consumption. But it was nice sitting at a stop light, the engine off, and still getting nice cold air out of the a/c! Mercedes in Europe have diesel/hybrids - rode in a taxi that had it - very cool! I hope VW heads in that direction soon.
Back on topic: One consideration for me to NOT get the metal rear bumper protector is if I was doing it to protect against scratching the bumper surface, the metal protector would eventually start showing scratches as well after dragging luggage in/out of the trunk.