So, the point is, if your going to tune the car successfully you need to change ignition timing, fueling etc.
All road engines run leaner than the ideal mixture (Stoichiometric) in an effort to reduce emissions. The new BMW k1200 motorcycle runs so lean on a steady or closed throttle that it actually "hunts".
The ecu is programmed to match emissions targets at the revs at which they are measured. Without a gear sensor it reverts to these setting every time the revs match the measurement revs, so you get a "flat spot" in every gear.
I come from a motorcycle background where ecu's have gear sensors which alter the fueling/timing/advance to blunt the power in the first few gears to stop you flipping the thing. They also have knock sensors (on some)so they can run right on the edge of detonation
When the ecu goes open loop it reverts to its "core programming" as it were, of reacting to the sensors available. It can only react within this program. Introduce more air than this program allows or anticipates and .......
Too much air.
If ecu's were all that clever, all the tuners would be out of business. The ecu would change its program to suit whatever mods you put on the car and the chip/re-map community would die over night.
As a final word of warning/advice
If you do anything that changes the flow of gases into or out of an engine it is best to have it checked at a tuner/dyno to ensure that the injection system/carb has compensated.
)