you say it's lean but what do you think that means exactly?
what are the egt's like and is it detting?
if the above are fine then there is no problem, you want it leaner on cruise to save fuel.
as for on boost, well you want to head for whatever is safe and makes best power, this is different on all cars.
what wideband was used?
what injectors are being used and have they been cleaned?
what were the AFR figures?
mine runs 15.0 on idle is pretty smooth and produce piss all emissions, just seems to burn really well, not so great on economy up the revs though