I'm very confused about the correct use of the Advanced Steam Turbine; as far as I can tell the hottest steam K2 is capable of producing is 415 degC with only a single level of Heat Exchanger, but the Advanced Turbine seemingly can't operate at full capacity short of 975 degC.

Sure, they still have a higher cap than the basic Steam Turbine, but the cost is that they make the core engine's estimate of total power capacity completely useless since they report a higher capacity than they can reach.

So what am I missing? Are there better steam sources than the basic Heat Exchangers? Is there a way to increase the temperature of existing steam before feeding it to an Advanced Turbine? Or am I missing some fundamental steam calculation rules in the base game?

I did search several pages of K2 forums and the FAQ and built-in wiki, and I reviewed the base Factorio wiki online, but found nothing that would explain the gap. What am I missing?

They are used for the Fusion Reactor, which is the tech that they became available in. But using them on a nuclear setup is about the same as 2 steam turbines I believe.

You still get the 210% efficiency bonus out of the turbine even if you supply it with nuclear 415 C steam, so it's worth upgrading to the advanced turbine even if you don't have fusion. One advanced turbine consumes as much steam as two standard ones, but generates 4.2 times as much electricity (42 MW).

Ah, neat. I wasn't entirely sure if the efficiency worked that way, plus nuclear power basically only powered the fusion reactors initiation.