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it should be noted with a gasoline engine, there is a fairly direct relationship between engine airflow and fuel consumption. 1500RPM with a manifold pressure of 20kPa is pulling a lot less air through it than 1500RPM at 40kPa. probably not as low as an 800RPM idle at 40kPa, but it isn't nearly as dire as it looks.
also(since I don't have a position in the industry to protect), GM's term for this situation is(or at least, used to be) a "throttle follower" function. back in the days of a throttle that was separate from the IAC, a few variables went into the amount of additional airflow that was allowed through the IAC while driving, so that if/when the throttle was snapped shut, the engine wouldn't instantly react in a manner that would upset the driver/passengers. with a manual transmission, it was even more critical to prevent the possibility of a stall/near-stall while shifting. sometimes, it was overly boosted in that if you were to press the clutch in, the engine would actually accelerate, which is a weird feeling. now, with the IAC functions being dealt with via the DBW throttle, the PCM just doesn't completely close the throttle plate.
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