Combustion based energy generation is a pretty well understood process at this point - so we have very good numbers for efficiency. The absolute best efficiency

Combustion based energy generation is a pretty well understood process at this point - so we have very good numbers for efficiency. The absolute best efficiency we can get out of hydrocarbons - using combined cycle gas turbines - is about 60%. Given that iron would probably have to be combusted externally (similar to coal) you'd expect similar thermodynamic efficiencies - the best coal plants are about 42% efficient. So I'd expect the closed cycle efficiency of this to be at best 42% - assuming a 100% efficient regeneration phase (highly unlikely). It's probably more like 20% or less. Poor efficiency for energy storage isn't necessarily a dealbreaker - if the capital cost per kWh stored is low then it may still have a place in the mix - but it's unlikely to be a silver bullet.