I did the 50 yard rim fire challenge with a Bushnell AR 3-12 power set on 12 power. I had my 32x Sightron in the case just in case i needed it, but I didn't. Once i got settled in with the supports, found the right ammo, my Savage Mark 2 had no trouble performing, and 12 power was plenty to see the centre of what i was aiming at, and show if i was steady or not.
TO me, accurate shooting isn't so much about being able to see the fibres of paper in the 10x. Its about seeing the target well enough to know what center looks like, and knowing if you are steady. A good clear 4x scope can be enough to see your target properly, as long as your reticle doesn't obscure the target, but at 50 yards or more 4 power might not be able to show you how steady your hold is.
Personally I don't think magnification on its own matters much in your system, but works together with a variety of other factors to give you the right information so that your skill can take over and get the job done.
Like Grelmar said, clarity matters at least as much as magnification, but then reticle pattern, parallax adjust, etc also play a roll.
I think some of these guys using 24x + magnification on their rimfires are using those scopes because its what they have, not what they need. Most of the .22lr bench rest shooters I meet also shoot br or F class in larger calibres at longer distances, and just swap the equipment.