I'm to believe ash characteristics would be more of a product of moisture content/distribution, i.e. - proper drying and curing.
I had a friend growing up whose dad always talked about how you knew a Cuban (cigar) by the white ash. He always said, 'it's that island soil!'
Really, I think they just have a better climate and more stringent traditions in how they harvest, dry, and cure/ferment their tobacco.
I always thought flushing was used either to kind of 'wash out' any harsh fertilizing/pesticide chemicals absorbed by the plant (which, as I get older, doesn't really make much sense to me,) or to trigger the plant to push out some of its terminal phytochemicals that are more desirable in the finished product... which does make sense to me in a science that I'm not terribly versed in.