If I'm not mistaken, the voltage/frequency spec is for the quoted operating temperature, up to 85C.
You are mistaken it is not as easy as that. There are many factors affecting the speed of a circuit and temprature is only one. In fact it is the bottom end that suffers from a pure CMOS operation point of view but that is not the whole story.
There is no data because there is no simple corrilation.
So what makes it a ropy design is not that it is not an official board but the it is designed to operate outside the manafacaturer's specification.