The the theoretical maximum efficiency of any heat engine (steam, stirling, gasoline, diesel or gas turbine) can be calculated using the simple equation for Carnot Cycle efficiency, E= 1-Tc/Th where E is efficiency, Tc is the cold side temperature and Th is the hot side temperature (temperatures must be absolute, ie Kelvin or Rankine). From the equation it is obvious that hotter temperatures on the hot side Th will result in higher efficiency, so you are correct that 800 degree Celcius (1073 Kelvin) steam will give higher efficiency than 200 Celsius steam (a reasonable temperature for the Tenkiv nexus while still achieving good solar collection efficiency). But it is instructive to run the numbers to see what actual efficiency we are talking about.
We will assume a cold side temperature of 27 Celsius (300 Kelvin) for our heat engine.
At 150 Celsius above ambient Th (450 Kelvin) the max efficiency will be 1- 300/450 or 33%. So a perfect heat engine running on a Tenkiv Nexus collector temp of 150 Celsius above ambient (the Tenkiv Nexus will achieve about 65-70% efficiency in full sun at 150 Celsius above ambient) could achieve an overall efficiency of 33% x 70% or 23%. This is a bit better than very good commercial PV panels.
A heat engine running at 1000 Celsius (1300 Kelvin) Th could achieve a max efficiency of 1 - 300/1300 or 77%. So an ideal steam engine running at 1000 Celsius and with a condenser temperature of 25 Celsius could achieve 77% efficiency.
The reason we don’t see steam power plants running 77% efficiency (actual coal or natural gas fired steam power plants run more like 40% efficiency) is because the Tc is not 25 Celsius (its much higher), much of the energy released in the burning of the coal of natural gas goes out the exhaust stack (the combustion temperature of natural gas is about 1950 C so if Th is 1000C roughly half of the heat will be wasted (unless one uses a combined cycle plant, where some of this waste heat is recovered). A solar powered heat engine does not have combustion products that must be dumped so the losses can be much lower.
The bottom line is this: 1. A well designed heat engine running off of the Tenkiv Nexus could see 20-30% efficiency.
2. You need a heat engine designed to run at a Th that can be achieved by the Tenkiv Nexus at good efficiency (roughly 150-200C). There are ORC (Organic Rankine Cycle) and Stirling engines that are designed for these temperatures in sizes from a few milliwatts to many 10s or 100s of kilowatts.
3. You could also use a TEG (thermo Electric Generator) but these typically have very low efficiency (a few percent).