If done properly, a break-in can help a factory rifle. The problem is most guys don't do it properly. To do it properly you need to clean between each shot and get ALL the copper out. That's where most guys fail.
There's a good article done by Jim See in the Technical Discussion Forum...
Custom Barrel Care At 17X
It's geared to custom barrels but the principle is basically the same for factory barrels.
I Have broken in a couple of factory Sendero barrels and it did help, as in they are easier to clean now then prior. They were used when I bought them and real fowlers, and they still foul a good bit, but they are easier to clean now then before.
Theoretically, with a proper break-in, you should see longer accuracy strings between cleanings and easier cleaning. With an over bore like the 7 STW I would use light loads with a slow powder to save your throat... or you could just use up those factory loads.