The one thing everyone not addressing is the Mental side to playing at high levels at early ages.
The biggest thing high level, small field playing, accomplishes is set the bar for the mentality it takes to play all high level baseball.
Talented kids fold under the pressure, or learn early to make the changes to deal with the stresses and adjustments necessary to go to the next level.
Mental failure sends more home, than field size.
High level any age, points out the mental toughness needed to play high level baseball, and those that play early on small fields and survived, have this advantage... undeniably.