In my personal experience, I had great results with training game-shape. Last season, I trained game shape for 3 weeks straight. In the first week, I had all of my starters and reserves with game shape 9. In the following 2 weeks, only 1 or 2 starters had an game-shape 8, but they had 82-83 min played.
As I know, training game-shape is only usefull for 1 week, and with every following week the persentage and effectiveness of the training falls down. That is why I suggest using it only before really important games.