Third Hand Information Alert (No, I have not experienced this myself, but "I heard tell!"}
Polishing most barrels is just fine, but, if you have a smooth twist type barrel such as the FX or I believe newer Daystate barrels, polishing can actually increase drag instead of reducing it. The smoother bore in these barrels actually increases the drag on the pellets. Go figure. Do be careful.
End of Third Hand Information Alert!
Now, as for pellets polishing the barrel, I have no opinion, but I will say that Mr FX (Fredrick Axelson) did tell me himself (UAG Grand Opening last November) that my brandy spanking new X Barrel would need at least 500 rounds before it was broken in. Mr. Ted Bier told me a few minutes later that he thought his guns were getting broken in at about 10,000 rounds. I haven't shot that many rounds through both of my rifles put together yet.
I assume breaking in, means that the lead polishes the barrel just a tiny bit with each shot. It's more than leading up the barrel, as breaking in must wear the barrel to a certain extent to overcome machining imperfections, though they may be incredibly small imperfections.
Just sayin!