[Air-Hybrid]

Harlan:
"Also injecting steam allows you to recoup exhausted heat and preheat your intake at the same time."

Surely this flies in the face of the principles underpinning a Heat-Engine (Carnot).
The only true efficiency gains of HAIs is that improved evaporation of the fuel in warmer air allows for a cleaner burn.

Dumping heat from the exhaust into the intake has (by Carnot) to lower the thermal efficiency of the cycle.

So only in the circumstances where the gains arising from improved fuel evap. outweighs the efficiency losses caused by lowering thermal efficiency should WAI be used. And then the system should only ever be driven by coolant heat - because disposing of the heat in the coolant has it's own efficiency wins (reduced rad opening/aero and coolant pumping), whereas exhaust heat is actually best serving the engine if remaining as hot as possible.