Yes- MMP depresses home runs and slugging percentage for left handed hitters, leaguewide. Berkman in particular has spoken about that at least 2 or 3 times, that it pisses him off when people talk about HR's at MMP being easy when in reality, for lefties at least, or non pull hitting right handers it is difficult.
Also, the ball, I think, carries much less with the roof open then with it closed.