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Great answer from pgfpro.
Most cars lose a lot of efficiency under boost because of two things:
1) They need to retard ignition timing.
2) They need to run rich.
They also lose efficiency overall because 3) they need to have lower static compression ratios.
All three items are to fight detonation. Additionally, higher intake air temperatures also make detonation more likely, resulting in the need to pull more timing and add more fuel.
Running a sufficiently high octane fuel would fix this. Lean burn additionally reduces combustion temperature.
Get detonation under control, so you can avoid running rich, and keep spark timing optimal, and you should see decent efficiency gains both from harvesting exhaust energy with the turbo, but also from increased cylinder pressure and effective compression. Low static compression ratios can be worked around by running Miller cycle (effectively Atkinson cycle + forced induction).
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