I don't have any Swingle to offer you, but my 2 biggest and prettiest trees out of the 50 or so that I have, are on US852. I am looking for seeds for them. I am in zone 8a in west central Ga, and all my trees are in the ground. I have many cultivars on different rootstocks in trial, and so far US852 looks the best. I don't know about productivity yet, as they have only been in ground for 2 years. The two trees on US852 are at least twice as big, if not more, than any of my other trees. Hope this helps. BTW, I think Stan McKenzie has seeds for both.
TFN