@_wtf_

I think running a full scale steam piston engine at red-0heat levels in order to get the gas expansion rate (and hence rev rate) up to what you would see in an IC engine would be pretty near impossible to achieve reliably at full scale. High powered IC engines tend to use liquid cooling of the cylinder walls in order to keep keep the lubricant working and stopping the engine seizing up. If you use re-hot steam in a reciprocating engine and cool the walls down to a reasonable temperature, then the efficiency of the whole thing is going to suffer horribly. Thermodymic efficiency would suffer even more as it would effectively rule out using a compound engine (even if the cooler cylinder walls didn't slow down the steam expansion in the primary, the exhaust temperature is going to be much lower so the expansion rate would be slowed down).

There are very good reasons why steam piston engines tend to run at lower temperatures and lower RPM than IC engines. Even if somebody can get a small scale steam piston engine revving into the stratosphere, that isn't going to be a practical thing at full scale. Keep the really high temperature stuff to turbines where you can run them, hot as there is no direct contact with lubricants.

As far as thermodynamic efficiency goes, I think that for ship propulsion at least, a diesel engine is still the thing to beat. Thermodyamic efficiencies of over 52% have been demonstrated where the best steam turbines are still below that figure. In commercial shipping diesels tends to dominate over steam turbines (the various navies of the world have different priorities - especially where their source of power is nuclear).