A 5*Player with the Skill "Speed" at 210% gains as much as a 5* Player with only 90% Speed-Skill
Is that correct?
yes correct
proof ?
many (18 yo) young fast trainers, after 2-3 seasons of power-training, usually have abnormal skills
For example a ST could have 300% at shooting/ finishing and 30% at tackling.
So yes, he will have the same progress with a new 18 yo (same q) fast trainer with balanced skills.