have YOU never heard of tuning before?
you may be able to run moderate levels of boost on stock internals, but to make any real power, changes need to be made to the timing, and compression of a naturally aspirated engine in order to not detonate under boost, so unless you have some magical system for modifying valve/ignition timing, fueling, and compression on the fly, you will lose economy 100% of the time, regardless of whether or not you're in boost.
and unless you're running an aftermarket ecu, the factory computer chipset on a n/a car doesn't have the flexibility or the correct inputs to measure a boosted situation, so your fuel map isn't flexible enough to have full variation for fueling in boosted and non-boosted situations, so a comprimise must be made. You have to fuel the car somewhere just rich enough to not blow your engine under boost, and just lean enough not to stall or spew out tons of smoke and foul your plugs when you're not under boost. Neither of which are ideal for economy.