True, hypothetical but based on experience, once you get the right blend (ratio), you should stick to it. I have found that if I suddenly make a sudden change i.e. increase a certain attribute too much and screw the ratio, player performance start to change.

For example I have this one AMC/ST player which i assigned X, X-3 (Phy), X-6 (Def), and he was doing well, then I decided to make him more of a striker and started X, X-3, X-15, he suddenly doesn't score/assist and gets low match rating, even a 4. So it seems that once you get the right blend, you shd stick to it.