if you win a level, your 6* players will loose a star and will win one year of age, so, yeah, they will train a little bit faster because they lost a star. Aprox. when a player win a star the % of proggress that win a player in training is reduced a 30%.
In the case of the 8* to 9* I can say a 50% looking that my player Cocu won +30%in hard training match practice and now won 15%, so, as all players Cocu will have 8* again next season and will recovery speed training again. Maybe not all, but, will recovery, and more reason for that if is young.