interesting one this...
on light cruise conditions, providing there's no Cat's to keep happy, engines will in general happily run quite a bit leaner than you are forced to have under lambda control.. Closed loop lambda forces fuelling to Lambda=1 (14.7:1 AFR), normally required to keep the Cat happy. This is a good bit richer than the engine needs, so you would think that it should be feasible to see better off-boost economy from an open-loop map. However I suspect the answer in this case is down to resolution limitations of injector pulsewidth in the map, which in this case is helped by the lambda control strategies....
Stu? Simon? Gareth?...