I plan on going with 26x9 all around as well. To keep my RZR under or as close to 50" wide. As far as 12" vs 9" wide tires, it all comes down to contact pressure and when you need it(hard pack, rockcrawling) or not(sand). Mud is a toss up do to what your riding in and the need for flotation or control. Check out Highlifter forums and look up "narrow tires vs. wide tires".
One time I was trying to get up a hard packed hill in my rockcrawler with 12" wide tires and kept sliding off the hill. Da Wife drove right past me in her street 4x4 on 8" wide tires, do to the higher contact pressure. Look at some ice racers tires, yes they got studs, but their tires are not all that wide. Look at racing swamp buggys in open class, no wide tires, need to dig to the bottom and control the turning. On the other hand, look at mud boggers open class, wide paddle tires to grab as much as possible. But little to no control.
Are 9" wide tires the answer to everything , no, but Da Wife street 4x4 as driven all over Pismo beach with out much of a problem.
I'm just saying.