I read a while ago (although I can't remember the reference) that heavy rain can add up to 2% in aircraft weight, and up 20% in drag!
As you would know...... since V2 and Vref are scheduled in accordance with the stall speed, which would increase in heavy rain, perhaps consideration should be given to raising these values.
Although I don't know of any SOPs that do.
Not very helpful.... I know.