That includes this year and September and October of last year. He has .728 this year. The AL average is .409 this year. So Davis is 78% better than the league aveage. That would be the 17th best ever. See Chris Davis Has A .722 SLG Over His Last 302 ABs. He is actually slugging .747 in road games and .711 in home games this year. But according to the Bill James handbook, Camden Yards has a HR rating of 129 for lefties over 2010-2012, meaning their HR rate is 29% higher there than average. So he could be helped by his park.
He also has a 202 OPS+ this year as does Miguel Cabrerra. The last time there was a season with 2 guys getting at least 200 was 2001. Before that 1994 and before that is was 1961. Prior to this year, there have been 59 seasons of at least 200 in OPS+, going back to 1876.
Click here to see the all-time leaders at Baseball Reference. Here are all the years when at least two guys reached 200 in OPS+.