The first thing to mention is that these grades are all manufactured identically, and their performance is measured so that they can be put into one of these three bins. There might only be two real bins, but at this point it's speculation.
The temperature plays a part in the speed as well - the higher the temperature, the lower the maximum clock frequency. At cold temperatures, the chip will run faster. Cold temperature failures are usually hard failures, in the sense that reducing clock frequency won't fix things. "Worst case" depends on your application. Here are a few scenarios that could happen.
- The PLL could fail, and the chip would not operate.
- The memory could enter a failure mode (can't write or can't read reliably)
- Data corruption due to datapath hold-time violations
- Improper behavior due to hold-time violations
- Excessive EMI due to uncontrolled transitions in the output busses
There is a distinct possibility that they only qualified the industrial temperature range part because a wider specification means more time and money. In that case, all grades will probably work down to -40C. There may also be more durable packaging with the industrial range part.
If you are using this part for a hobby project, you may be comfortable "risking it". You may also be able to qualify individual parts, but any manufactured device will be a tough sell without the wide temperature range chips.