What risk?
I asked a bio lecturer at uni about this and as long as it's going to stay above pasteurization temps (72c or above) for 2-5 minutes, all bacteria (except some extremophiles that even boiling wouldn't kill) will be dead
What makes you think that everything would get to 72C?
If you cool the wort down to 82C in your kettle first then you will also get further temperature drop when:
1. You put the wort into a room temperature plastic cube (probably loose about 2C)
2. Then the process of filling up your cube will probably take about 3 minutes (probably loose about 2C)
3. Then even though the main body of wort was 82C you have to look at the coldest part of the cube to ensure pasturization. So most likely this will be the lid (or if you have a tap fitted quite likely this will be an issue). Let's say the cap of the cube this is the last piece to go on and you might find that the seal in the cap will never get to 82C and it might actually never get to 72C.
So even though you only cooled your wort to 82C in the kettle it's quite likley that you could get some microrganisms in parts like the seal in the cap that might only be exposed to lower temperatures. So the risk would be that if something only went slightly wrong you would not have any buffer and might potencially have an issue. We have seen fresh wort kits have issues before for this same reason and they are generally filled very close to boiling point. You may as well fill the cube hotter and be sure. If you are really concerned about isomerisation in the cube then just chuck the cube into a larger bucket of water once it's filled and it's sat hot for a few minutes.