> For this reason you usually have a large imbalance of one propellent to the other to keep the temperature down.
So how large is that imbalance usually, or in the case of the Merlin 1C?
> although the Space Shuttle Main Engines are a US-design example of Staged combustion.
Is this, why their exhaust is so clean?
How much more efficient is this engine compared to the Merlin?
I'm excited, this engine will be used again in the new super heavy lift  that NASA is developing, though obviously, I can't tell if its a good idea.
The Space Shuttle Main Engines used LOX and LH2, which creates water when burned.
>How much more efficient is this engine compared to the Merlin?
The specific impulse is the metric to look at, and ballooney already talked about it.