Chris_W, since you have been dealing with both X-7 and X-9, though for different speeds, do you see any essential superiority to X-9? I do not care about weight particularly when its reduction were to bring vulnerability.

Incidentally, my long-term thinking about the chain slack, in the context of those derailleurs, is to build up the cage so that includes a third pulley like old Suntour. One positive aspect for me, with that crummy Sram cage, was that I could make one of my own and things could work perfectly, so I am not scared to move ahead. As to taking care of the slack, I have another tensioner system right now, external to derailleur, but through drivetrain changes recently acquired an extra slack that I would not mind eliminating.