I just picked up my SBEII yesterday. It is my first autoloader and I have the same concerns. I have heard to break these guns in with the heaviest loads you plan to shoot. But here is the actual quote from the manual. It is very ambiguous.
This is on page 49 of 118 of the online manual.
"WARNING: due to precision machine tolerances
on your shotgun, some breaking-in
period may be required before your new gun
works perfectly with light target loads. If you
experience any initial functioning problems,
we recommended firing three or four boxes
of standard hunting loads to allow for this
break-in period.”
I plan to shoot heavier loads to break it in but it would be nice for Benelli to be a little more descriptive.