There are a few models which replace the ducting, and block off the airflow to the oil cooler on the left side. Any unit which utilizes the stock bottom airbox (now the most popular of solutions, as the aftermarket figured out the stock bottom/duct is awesome) keeps them intact. The IE intake replaces part of the ducting, but leaves the left side open to maintain that circular airflow into the engine compartment.
Being as there seems to be little if any difference between most of the intakes on the market, and the fact owners making over 500 hp+ (440 whp) are utilizing the stock inlet ducting, I don't believe it poses any sort of restriction to the intake system. That is just my opinion based on available data. (I always reserve the right to change my opinion when new data is made available!)