don't have the guide of power training star by star... I know that with 4 stars player train better that with 5*, 5* is when start decreasing in trainings... and if... I remember... 1 year ago when I had 3* players :'D LOL and I think with 3* train slow than 4 so, to see if "train good" the potential that you are mentioning, you have to know that a good trainer will win +120-150 of progress in training at 30% max when have 4*

the value and wage, as fugazi said, is important because the wage is increased when you train a player, so in the market fast trainers are for sure players with the Value = (X)more times the wage, too, is something normal that fast trainer have more value than other players.

As you know 15 cones, = +1 of Quality and 5 Quality points = 1 star, if you go to my team page in the link of my signature I think that I have a academy player, obtained in December of 4*, now have near 7* defense 7* attack 11* phisicall/mental skills.
Remember that academy players of 4* and Nordgens obtained when the nordeus day start are fast trainers.