Since 2006, 851 players have logged at least 200 PAs in the Florida State League (High A). This is a sample of players that includes guys like Giancarlo Stanton (age 19), Jay Bruce (20), JP Arencibia (22), Allen Craig (22), and Domonic Brown (21).
Another 746 have logged at least 200 PAs in the Eastern League (AA) since 2006. Players include Brandon Belt (22), Matt Wieters (22), Pedro Alvarez (22), Ike Davis (22), Kevin Kouzmanoff (24), Josh Reddick (22), Carlos Santana (23), Brennan Boesch (24), and Adam Lind (22).
Miguel Sano has logged 200 PAs in both leagues. He's also put up the single highest ISO since fangraphs started tracking these statistics in 2006. In BOTH leagues
(ISO, or Isolated Power, is Slugging Percentage minus batting average. So it measures pure power - how good a player is at accumulating "extra" bases in the form of doubles, triples and home runs.) Here's a quick look at the top 50 performances, by ISO:
Five players posted .250 or better ISO seasons at age 20 or younger in the Florida State League. Of these, only Giancarlo Stanton went on to post a better ISO in AA as a 20-year-old in the Southern League (although Javier Baez is holding a .352 ISO through 194 PAs, also in the Southern league).
As stated above, the sample includes all players with a minimum of 200 PAs in either league. If a player hit a lot of gap triples and bloop doubles, ISO could be measuring speed more than power (ISO is Slg%-Avg, or total bases, not including 1st base, divided by ABs). In order to eliminate that possibility, we can just look at home run rate, expressed as AB/HR.
Sano ranks 2nd and 4th in home run rates (tops for the EL) since 2006. If you have been following Sano's year, this shouldn't be a surprise - he has 31 bombs after all. And look at the ages at the top of that list. One of these things is not like the other.
So there you have it. Sano's 2013 ranks among the top power seasons in both the Eastern League and the Florida State League dating back to 2006, in just about any way you care to measure power. And he's done it at a younger age than anyone except Giancarlo Stanton.