IMHO, regardless of frontal area being exposed to airflow, unless there is a substantial pressure difference, you wont get effectively cooling. If for example (this is exaggerated to show a point), you had the entire front bumber assembly ducting air to the intercooler (lets just say this is the only heat exchanger), you would now have a HUGE high pressure zone infront of the core. So, in theory, this would only aid in cooling as the low pressure is more likely to "suck" the air through the cooler.
But, as we all remember from physics, nothing gets "sucked" into a space, it gets *pushed*.
As long as the low pressure area behind the cooler can be maintained (read: air flow escaping the engine bay area) then this ideal cycle can continue. And, as you said GTS, the real world case of stacked cores has to be addressed. So in this instance, ideally you wouldnt want 100% of the air going through the front of the car through the intercooler followed by radiator, as the accumulated hot air could harm this whole process.
I wavered from my initial point a bit, but you get the idea.
Pressure difference and maintaining escaping air flow is king.