The only CF hatch I've ever seen was a custom, one-off piece that a guy paid a lot for. if you've got that kind of cash then worry more about building the engine for more power and less about losing the weight. Reducing weight from the MKIII is not an easy feat. the hood and hatch weight a lot less (relatively) than you may think.... the hatch glass (which you'll still have in a CF hatch) is the heavy part of that piece and you'll loose more weight than you would from swapping the hood by simply replacing the drive shaft with a lightweight, one piece DS.... AND it's a reduction in rotating mass AND it's central. Lighter rims; again even weight reduction AND it's rotating mass.
Oh, and if your car is a targa top... trade it in for a hard top... they're lighter too.
As for the spare... get a AAA membership and ditch it except for when you're going on long trips. Hell, the last time I needed to use a spare it was dry rotted to hell and had no air from lack of use. I do still carry one, but only becuase I don't have anywhere to store it outside of the car.