Here's a summary of some training averages from a few of my guys last season. 5* = 40Q.

I've found that:

the variation between Highest and lowest gains is around 60%.
Drop in gain per star increase seems to be settling at around 25%.

Nothing earth shattering

Training Log - Intensity = Hard

Baxter 15T Academy Player MV 37Q=7.3; 46Q=12.7; 47Q=13.5
4* 47/96/140
5* 36/75/116 - %age change from previous star = 78/78/83
6* 29/58/89 - %age change from previous star = 79/77/76

Mackay 5* Market purchase MV 43Q=10.9; 46Q=12.9; 48Q=14.2
5*45/71/115
6* 28/60/88 - %age change from previous star = 69/80/77

Hutton 5* NordGen MV 45Q=10.2; 46Q=10.6; 47Q=11.4
5* 38/79/117
6* 28/59/87 - %age change from previous star = 74/75/74

McGrain 5* Market MV 44Q=10.6
5* 35/76/120

Gemmill 5* NordGen MV 47Q=12.3
6* 33/67/103

They are based on relatively small samples of around 20 x Stretch, Cardio and Match - its hard to get bigger samples unless you train roles to hold them at their faster rate for longer.

As far as high and low values go the first player listed achieved from 108 to 175 on hard match practice.