Here's my opinion on the star system in training. I have no knowledge of the game code and base this only on my own limited knowledge of programming so someone with more expertise may well read this and poke a thousand holes in my reasoning.
Writing code is all about not only what it does but how it does it. FM is essentially a huge spreadsheet involving thousands of calculations. The lower the number of calculations necessary at any given point to reach a desired outcome then the more efficiently the game will process. Basically less calculations equals less processing time.
Consider how the game theoretically (to reiterate I have no knowledge of the code but am making an educated guess) might use training within the player development model. Everytime the player development model has to calculate by how much a players attributes increase it has to reference the quality of the coaches and training facilities. If instead of continuously doing this, assign a star rating which is updated on a less regular basis. This value gets stored and is then referenced directly by the player development model. So instead of having to make say 10 calculations it only has to make 5 thus a faster processing time.
Based on this logic I have always assumed that the star rating is the key and have had no obvious problems with player development. Of course I could be completely wrong and the star system might be there just as a usability enhancement to make assessing training quality easier for the user.
Just my theory on it anyway