I had a 18 yo MC last season I've mentioned him a few times in other topics. he was a few SP from scout.
I allocated the SP as they were awarded.
As a five star he trained at around on Avg at 140 progress per day after allocation and gaining his 6* that dropped to 111 these figures were obtained over a 10 day period at the end of last season.
This season back to 5* L6 he now trains at 163 progress again figure calculated since day 1 up to day 8
Match/Games P8 + 1sub gains 957
Practice games P5 gains 254
Cardio 2 sessions 61
Stretching 2 sessions 33
He missed one day of training through injury.
As others have suggested and IMO this goes a long way to proving the theories, do not add any SP that will improve * rating.
The above increase also apply to older players pro rata to their own previous gains.
I'm also finding up to 30-50% variants in training gains depending on age and when they actualy under take training.
Training in the morning before any games yeilds the 30-50 drop in progress, highest gains are after a match and especially so if the player has been given MOM where I'm finding up to 25% gains over the Avg.
All the above is on Normal intensity.