i edited the youth rating for a number of nations prior to starting my last game. i was surprised to find that lowering the ratings for brazil (who normally produce too many world class regens) didn't really reduce the number of top quality regens they had coming through by a significant amount. however, i increased the ratings for spain, portugal, holland, england and several other nations and they all do seem to be producing more great players, so clearly changing the ratings does have some effect.
i also reduced the youth rating for argentina, mexico and turkey, which has led to them all producing fewer top regens - however, the latter two are still producing more excellent players than they should be. for instance, i reduced turkey's rating to 105, yet in 2026 the majority of their first team play for the top teams in spain, england, italy etc.
it's very difficult to get the balance right, especially as different people will have different combinations of leagues, players loaded, etc for their games. however, ideally si should probably reduce the frequency of top regens for brazil, mexico, turkey, argentina, egypt, algeria and saudi arabia, as they seem to be the nations who get too many top regens.