Couple things that come to mind:
Strong magnets would certainly cause feed issues in semi auto guns. One little pencil eraser sized rare earth magnet could easily hold 10+ pounds. If the magnet was anywhere near any steel in the shotgun it would likely be attracted with enough force to cause a stoppage.
I would think the pattern would be somewhat sporadic because A) not all of the magnet peices would stay together, and B) the magnets attraction to the barrel could affect the trajectory as its leaving the barrel. Granted with the magnetic shot moving at 1,000 fps+ the effect will likely be small, however even 1\10th of a mm near the muzzle will result in feet at exdended ranges.
PErsonally I would like to see what happens, however I find it to be pointless. I say that because A) it would be probably cheaper to shoot gold plated steel shot, B) If the magnets don't seperate shortly after leaving the muzzle, you are going to have a mass that is likely shaped like a turd (And who knows where its going to hit) and C) The shock and deformation caused by the charge could possibly affect the magnets negatively and reduce their magnetic attration.