I've taken all two of my birds with a Mossberg 535 and the full choke that came with it. Both of them dropped in their tracks, but unless you may have to hunt some wide-open terrain or you want your turkey gun to double as a goose gun, I wouldn't worry about getting an ultimag.
I will say that two advantages that Mossberg has over Remington are the location of the safety and sights. If you're used to the trigger guard safety of your current set-up then it isn't a big deal, but having the front and rear sights on the barrel make headshots a snap!