A 9-inch twist would do well if the quality of the barrel is high. The 9-inch twist spins bullets at 288,000 rev/min at 3,700 ft/sec muzzle velocity, and 320,000 rev/min at 4,000 ft/sec. I don't know the spin limits of those particular bullets, but at least one other brand in that same weight range comes apart in midflight at a bit over 300,000 rev/min.
An 8-inch twist spins bullets at 333,000 and 360,000 rev/min at those velocities -- clearly too fast.
Looks from here like the 9-inch twist may be the only decent choice open to you. A 10-inch twist MAY work, but I'd hesitate to bet on it.
At the moment, the limiting factor is the bullet. Until we get some good .224 bullets with tougher jackets, trying to get its full potential out of the .22 WSM will be tweaky at best. I brought this problem to Jimmy Knox's (JLK) attention in 1997 or 1998 but never heard further from him about it. He may be making .224s tough enough to handle higher spins -- I just don't know.
