No, I haven't, but I spend hardly any time on the highway these days. I've only just begun to dabble with this. But from what I've seen, it's near 28 IGN for the speeds I drive (perhaps admitting more than I should...).

If you read the thread over at CleanMPG on Super Highway Mode, it seems the Prius likes 14 IGN. Somewhere in that thread is something about 28 (or 14+14, or maybe 2*14) but I had trouble following the discussion, and haven't really been able to figure out what that was about. But if they were talking about other cars, then the 28 for me (and you) makes sense.

What are your tires at? If you're looking to best your mileage, consider going to "max" sidewall, at least.