1- when you win a star, training speed is reduced, so 3* will train faster than with 5*. Usually if I remember is reduced between a 15% per star.
What count here is not the level of stars, thats a visual information, and thats not a game of maths in what +1 of quality = a victory.
Players are proggrammed internally with % of effectiveness and your work as manager is to test these players and decide as a real manager do.
So a 3* player can do more for your team than a 5* or a 6*? yeah.

True is that + value = fast training, so is interesting to buy players taking a look at the value and too the age, because between 18-22 age is when they train fast.
Next season if you promote players will loose a star, and with that will win speed in training despite the age winning, so take that into account.