For a comparison, by the end of May, 1959, Harmon Killebrew had hit 18 home runs. Although the old 154 game season started later -- about mid-April -- Harper also has 18 homers. He might hit a couple in the next two days, but most important: Harper is hitting at about the same power-level as Killebrew in Killer's "breakout" season, but hitting for a higher average, drawing more walks, and fielding well.
(Might have to compare Harper's walks to Eddie Yost's walks. If they are remotely similar, then the kid is playing at "Mickey Mantle level")