I've been wondering something similar. S. autumnalis has done well for me along a sidewalk here in 6a, but it has afternoon shade and the last couple of years have been wetter than usual. I have some more that I'm thinking of planting on the south side of the same bed, next to the driveway where it will get more reflected heat and not much shade. We'll see how that goes.

Full sun in a hot humid climate doesn't seem to work well for short matrix plants. There's a thread in the Perennials forum that dbarron started along those lines:

http://forums2.gardenweb.com/discussions/4861074/planting-suggestions-for-low-growing-plant-in-dry-conditions?n=19