Upper 80s really only happen in the Persian Gulf, that's the only place in the world. It does hit low 80's on the Eastern US coast, as well as in parts of the Midwest like you mentioned. (And anywhere in the ocean where sea surface temps are well in the 80's).
I think there can't be an optimal dew point. The lower it is, the more efficiently sweat cools you (because it evaporates faster). This would also depend on the temperature. At 65 degrees, a dew point of 15 would be an amazingly refreshing day...probably bright blue sky, a breeze, basically perfect running conditions. At 110 degrees, it'll be uncomfortable even with a ridiculously low TD like that. So basically the lower the dew point the more comfortable it feels, but that doesn't mean it's necessarily comfortable.