Perhaps I'm not understanding the mechanics of heat correctly, but it seems to me that vanilla heat pipes/heat exchangers have a maximum reachable temperature of 1000C, and the Cerys Reactor seems to have a maximum reachable temperature of 2000C.

Excluding the fuel consumption of the reactor and other mods adding buildings for higher heat, why is the reactor maximum heat 2000C? Are there any benefits/drawbacks to heating it up past 1000C, because I haven't noticed any.

The number was chosen largely in a vacuum. If heat pipes max at 1000, it makes sense the reactor does too.

Are there any contrary opinions?

I think higher temp on the reactor makes sense. When you first start using it and don't have much fuel, having the 2000C limit lets it stay hot without fuel cells for longer.

It is possible to adjust the heat capacity such that 1000 degrees stores twice as much heat as it does now.