Will grafting another variety on a mature tree produce fruit faster than on a seedling from the same variety ?
yes. grafting faster, and almost guarantees you get the same fruit.
Adam , I think I was't clear with the question , I'll rephrase it :
What would give the fastest fruiting :
Grafting on a branch of a mature fruiting tree ( different variety ) or
Grafting on a seedling of the same variety .
The budwood is from a fruiting tree .
And when is the best time to do this , it is an excellent White sapote for an almost tropical climate .