I've shot 3.5" shells out of an 870 Supermag, a Benelli Nova and a SBE... and honestly I couldn't tell much difference at all in the recoil from any of them. They all kicked the snot out of me. The SBE seeme to have a little less muzzle jump than the others though.
Most of the time I'm about two steps behind the wadding when I pull the trigger on a turkey so that may be why I've never noticed the recoil while hunting with 3.5" loads.
Just make sure when you go to pattern your gun that you don't lean up against something with your shoulder when you shoot. Made that mistake before with a 3" turkey load and learned my lesson. Also make sure the butt of the shotgun is resting on the meat of your shoulder and not down on your arm or up on your collar bone.
If nothing else, you can always put on a good life jacket and wear it while shooting magnum loads. I've seen a friend's wife do this when practicing with her deer rifle. :biggrin: