Ah sweet, I'd just figured the 9 was a newer version rather than referring to arm length. I'll be using shimano levers but sounds like the 8.4's will still work better with those for me, offering a bit more pad clearance at the cost of a slight loss of power
New fork looks sweet. Definitely add canti mounts on the back too.
CX9 work well. If you've got sram levers, using cx8.4 will give better pad clearance