The Bears' Mark Carrier was the last safety to win defensive rookie of the year. And the vote was not even close.
On paper, Caleb Downs is as good (if not better) than Carrier coming out of college. I can easily see him matching or beating Carriers' ten interceptions in his first year in the league. Like the OP said, the guy literally covers the whole field.
However, all great players are usually dependent upon their units in performing their ultimate best. My single hope is this season's defensive front getting consistent quarterback pressure on passing downs. If so, I am sure Downs will have a fantastic first season.