I've never ran a 9 twist, so I really can't comment from experience. From what I've gathered, it it's a true 9, there's a good chance it will work with the 75 A-Maxes at AI/22-204 velocities. It's a bullet's length, not weight, that determines twist needed.
My first barrel was an 8 twist Krieger four groove cut to 22", and the accuracy was a 3050 with the 75's. Ran them up to 3150, but was tough on pockets, and pressure seemed to spike quickly. It was a powder learning curve, not a dig on Krieger.
Next barrel was a 26" Rock 5R 8 twist, and the accuracy is at 3150, and have run them to almost 3250 with no pressure signs. Since this one's for SD prairie dogs, I'm happy at that speed. Still wanna test in hotter temps first, though before I load up a butt load.
I've got another 8 twist Rock coming in a #2 Douglas contour, for a lighter carry/calling gun in 22-204 I'm putting together, and I'm having it cut at 22".
As far as optimum length, it's always a compromise. You have to juggle velocity, barrel length, contour, over all weight, balance, and portability with all your parts as a total package deal.