Using lvl 6, i would say it is roughly equal to trainer with massage speciality, maybe slightly better. Only using for a season but effect is noticeable, but nothing drastic. If you want to play higher minutes, better have players with higher stamina.
Last season had average ~8,5 and maybe abit higher of game shapes whole season long.
This season starting lineup for example:
Stamina 8; Minutes 67, 70, game shape 8, 9.
Stamina 9; Minutes 87, 91, game shape 8, 8 rising DMI.
Stamina 3; Minutes 66, 64, game shape 7 rising DMI, 8.
Stamina 6; Minutes 59, 76, game shape 8, 9.
Stamina 9; Minutes 69, 89, game shape 8, 8 droped DMI.