This is anecdotal, but when I host my buddy at my place (he is high diamond on all roles) sometimes he'll hop on my account (low silver on all roles) and steamroll. I'd say 3-5 wins per loss, and he tends to stay off his mains. Yes bad games happen, but he consistently will leave my account at a higher SR than when he arrived (please don't ban me Blizzard). I am pretty confident a diamond level player could boost a bronze account in any role without much trouble.