Over on Insight Central, Julian Edgar is running an aftermarket engine control system.
Turbo, w/a intercooling, alternator and MoTeC management - Insight Central: Honda Insight Forum
This is leaving out a lot, but what he found was that running very lean was actually
very difficult, especially the transition in and out of lean burn. His mapping doesn't go anywhere near as lean as the Honda factory ECU because it resulted in a very undrivable car. You might want to sink your teeth into his thread though, as there are a lot of other factors, other than air fuel ratio, that could help economy.