Jeff, I don't know if this will be of any help to you, but we have a Halesia diptera (which is very similar to H. tetraptera) planted in full sun conditions and in hard, red clay. My USDA zone is considerably different than yours, however. I think I might be tempted to give it a more protected site in your location.

Our tree has done extremely well throughout these past few years of terrible drought, with very little extra watering. We don't have an irrigation system, but hand water our trees, etc. The flowers don't seem to last for very long, and the large foliage is subject to some wind tattering (not much). We live in an area called the 'windy knoll' for a reason.

Our tree was quite a bit larger than yours when we bought it and has been in the ground for three years. It was hit very hard by that wicked freeze we had last Easter, but came back beautifully.

One little observation that you might enjoy: our tree is planted fairly close to our back patio and has become a favorite perch for hummingbirds. If we wash down the foliage real well in the late afternoon, the hummers will take little baths in the leaves. It's adorable.

guess i will rethink that spot for the tree, rhizo. some things that have been planted there did well but most were very tolerant of the heat and sun (goldenrain tree and pampas grass to name two; both gone now).
i think i overlooked another spot that should be more to its liking - bright shade with a couple hours of sun around mid-day. i can enjoy it there from the carport.
cute story about the hummingbirds. :-)