The banana slices and sweet potato strips were still chewy after 10 hours at 158 degrees, so I let them go another 10 hours overnight at 95 degrees, and now they are crunchy.
I can't discern any real difference between the precooked and raw sweet potatoes. If I "squint," the precooked ones might be a tiny bit crunchier, but it's not a dramatic difference.
I shook a little salt over the sweet potatoes before dehydrating, but they could use more.