As far as I know, unbuffed (but assuming you have 5/5 precision), and attacking a boss, the soft caps are as follows:
Specials = 99
Poisons = 315
White = 722
With anything over 315 being "wasted", as compared to putting those stats towards things like Agi, AP, Haste, and ArP...
That's going by the info I can find on Elitistjerks.com anyways...