Question for the Power Trainers

[SimplySimon]

OK, quick screenshot of my entire squad after stretching on normal intensity:



You can see the gain and the ages of players from that - so my question is, who is a good candidate for power training and who is a slow trainer that I should consider letting go of?

(it's worth mentioning that Colman is my only 4*, so I guess he's training better because he's not as good as the rest)

As a rule, what rate of training should I be looking for to consider someone a 'fast trainer'?

That's two questions now, oh well

Thanks in advance for your help! :-)