Just thought I'd come back to this thread with what I've found; the
Sigma Sport ROX 5.0 Cyclocomputer does display both Average Speed along with Cadence and HR, too.
After all the trouble of finding it, though, I'm probably going to go a different way; MapMyRide (MVP) with my iPhone and Wahoo sensors/Rflkt. I love the MapMyRide app and didn't see any way of uploading data to it from the ROX 5.0.