Most guys shooting steel (ducks, etc) advise opening up one or possibly two choke sizes. However, that's normally with the larger shot. I'm not sure how the smaller 7 and 8 shot reacts, so a few shots at a pattern board will tell you a lot.
personal observations. Sure they'll run steel for a while, but so will a wallmart mossburg. You want a gun that'll take a lifetime worth of pounding steel? Look no further than the 870. Guess it just depends on how much you actually use it.