Got all cunning, not sure about diminishing returns, but atleast one pic i saw showed it way above 2500.
If you put all purple augments, power gives you:
252*0.17=42,84 bonus healing, and cunning gives you
252*0,14*1,05*1,09=40,377 bonus healing (sorc buff, 9% extra cunning skill)
so you lose about 2,5 bonus healing, but it nets about 1%(?) of crit chance. Not sure though, but the difference isnt too big anyway.
And that 252 is from having every piece augmented with purple skill augments. Late in finland, so fix my numbers if needed