Trevor,
think of what you just wrote. All the air is metered, right? So the PCM knows how much air goes in, right? Where is lean condition coming from. Besides, it's not forced induction -- the amount of air breathed in is determined by the engine vacuum.
There are other reasons not to do things like that. See Blorton's thread for one. The other would be resonant frequency of the long/short runners at low/high RPM.
