Assuming carbon based life as we know it, 300-400C is the probably a hard limit at which point single carbon bonds begin to break down.
Assuming life on earth as we know it, with ATP universally conserved across the entire evolutionary tree, the limit is really 150C. We've seen incredible survival adaptations like cryptobiosis but no organism exists that doesn't use ATP as its most basic unit of energy storage. That's the theoretical limit, but realistically other highly conserved critical pathways start to break down well before that temperature.
Carbon-Carbon covalent bonds are the strongest we know. 300°C is nothing, carbon melts beyond 3000°C. Some organic molecules in our cells degrade at higher temperatures, but it depends on the availability of other species to react. Otherwise, hydrocarbon chemistry can be done beyond 300°C.
I am not saying that this kind of organisms are likely (I have no clue, it’s not my field). But from a chemical point of view there are 2 factors that could be at play:
- exotic organisms could rely on different chemistries altogether to produce energy and rely on molecules that are unstable under our standard conditions;
- and also chemistry under pressure could be very different from what we are used to on the surface.
You can fantasize about exotic biochemistries all you want, but what we know about carbon based life here on earth is rather specific in its limits.
How about the organisms living in or around the hydrothermal vents in the mid-ocean ridges?