Steam turbines have row after row of blades, each successive blade wheel larger and lower pressure than the last. Exit temperatures from modern turbogenerators are around 120° C, where the steam is about to turn into water, and pressure is usually below atmospheric. Almost all the energy which thermodynamics permits to be extracted has been extracted.
I probably should have phrased it better - I was picturing [Furnace] | [Thermophotovoltaics] | [Boiler], so the PV part gets first crack at the light/IR from the furnace and then the boiler cools the PV gear as it boils the water. This is also how it's suggested to be used in a paper linked elsewhere in the discussion above: https://www.sciencedirect.com/science/article/pii/S030626191...
If you make it harder to cool the turbine by putting those neat the radiator, efficiency will go down; if you make it harder to keep the heater hot, by putting those near the fire, efficiency will go down.
As animats already said, turbine efficiency on power plants is very near the theoretical optimum. The only way left to improve it is by increasing the temperature.
The furnace burns at over 900C but the superheated steam (which is your ‘hot’ temperature for your heat engine) is only at ~400C, you might lose some power output but you won’t be losing efficiency.
