The early grafting advantage isn't a matter of having more growing season at all. And I'm only talking about pears and apples here. I think that you want the grafts to begin growth during the first flush. That is when the rest of the tree puts on the majority of its growth and I theorize that it also pushes much more excellerated growth of the grafts at this time. A matter of a couple weeks gives you tripple growth.
Peaches and Jap plums are indeterminate growers- they grow rapidly all season long. Any species that falls in this growth habit wouldn't benefit much from earlier grafting. This is my theory.