I did a lot of training a couple of seasons ago and I couldn't find any significant effect of condition on training gains. But that doesn't mean there isn't one.
With fast trainers its hard enough to get enough data because, if they are playing too, they progress too fast - so its even harder to get enough comparable data for a range of start conditions.
Some have said they see better training between 60 and 80, others different - I think this depends on the amount of variability you get and when you stop recording.
Just plucking an extreme example out of my data I have an academy player that at 4* and at 90% condition achieved 175 gain at Hard Match Practice: at 58% starting condition he achieved 171 gain, Hard Match Practice.
Imo, the MV of players is predominately made up of value for Q, value for roles, value for SA and value for training rate and with high value fast training players the largest proportion of value is for training rate.