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Here another example... Of course Digba Zah is a bit lower and that means he should gain more from the head coach, but 4.3% more seems a lot when he is only 2% lower initially.
Also Kouadio was a ST who costed 5T and Digba Zah was the free one. This means Digba Zah is the faster trainer? I think so, we could see next season if we test them with special ability. But if I wanted to invest in a coach for one of them or only keep one of them to not have 2 players counting for my Top 14 players average, I could release Kouadio as he should be the slow trainer here.

We need a lot of data to find out in what range the % gain is possible for a starting quality.