Not sure CF is worth the extra $. Get a aluminum for 600 vs 1000 for CF.
When I moved on to a GT350 I sold most of the GT500 stuff I had. I talked to Tom Wenstrom (the gentleman that was machining them for me) about carrying on the flanges as there were a few that were interested. The programming is done and it is simply a matter of there being enough people interested to warrant purchasing the raw steel billet and then machine time. Try him at Wenstrom Design if you really want one and you may just be able to convince him.
That said, if the aftermarket can't make a CF shaft for the GT350 that doesn't have issues I may get sucked into this again.