Running in the "gray" area means that the chip is not GUARANTEED to work, even though any particular one may. Another chip (even if it of the same type) may not work at all. The chip may also stop working at temperature extremes.

I don't know if one is available that meets your needs, but why don't you use one of the 18FxxJxx series chips? They are guaranteed to run full-speed at 3.3V.