Another test, this time a short sample when I pushed my 7 star 20 y/o attacker to 8 stars:

Ran this session 12 times:
Click image for larger version.  Name: 15 Övningar test 7.jpg  Views: 284  Size: 41.9 KB  ID: 71960

Got these results, the sample size was so small that it didn't really provide any good data on how much effect grey abilities have on the development and how the gain is redistributed.
Click image for larger version.  Name: 16varför.jpg  Views: 284  Size: 89.6 KB  ID: 71961