The thing about using props to grow hair: the density of the hair is in proportion to the area of the poly - a skull cap with four times as many polys would have four times as much hair. This is one instance economizing on the number of polys has a bad effect. But, it does have to be balanced with having a hair prop that is not so large that it freezes the Render. I think it also means that the prop should have polys that are as near identical in area as can be done.