Stockping sps is counted in the overall quality so would you pile 15sps your player would be treated as an already 5* one, hence his trainings would overall diminish in terms of gain (not much difference between q49 and 50 tho ie).
The only case in which your player would keep the same training rates is if you assign the sps to either SA or new role learning.
As i said, if you keep points or assign them to quality, they ll be counted the same way and your player will slow down proportionally, doesnt matter if you raise his q or mass points up. Everything not going into new role or sa slows down the growth rate. In the new season by losing his stat the player will re-fasten up, but dont forget he'll get a penalty by aging one year (from 18 to 19 is slight). Finally, even losing a star any player moving on a new age bracket will keep growing slower (>21yo, >23yo and so on).