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AFAIK engine efficiency isn't *horrible* as long as it's sort of warm, since the main problem with a stone cold engine is fuel not mixing with air and burning, perhaps sticking to the walls. The combustion chamber walls are extremely cold compared to the actual gases even at full operating temp. At say 160F instead of 200F, your engine clearances are okay and the fuel vapor pressure is high enough to burn effectively.
But yea having a water cooled exhaust manifold to transfer more heat to coolant also helps. It's a pretty standard design feature nowadays, since it simplifies the exhaust and reduces warmup time.
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