I had my youngest daughter shoot her first bird with a high brass 7.5 and it flat smashed him at around 20 yards, even broke wing bones. At that point, I was more concerned with having a forgiving pattern so she wouldn't miss then I was recoil. But like Pittboss mentioned, Hevi 13 6s or 7s in 2.75 are very light recoil and will give that extra little punch over lead if needed. My kids 20 shoots a very huntable yet kid friendly pattern out to 35 with 2.75 6s out of a factory full choke.
But yes, a good 7.5 lead will kill them all day long if the pattern is good and distance is reasonable .