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My understanding is the issue stems from the extra secondary intake ports. In stock form they are actuated; closed at low engine speeds and open at higher speeds. The S4 engines use exhaust pressure whereas the S5 engines use the air pump to actuate rotating valves to open the ports. My understanding is potential is limited because the ports are too big to be left open all the time and too small to be left closed. There are isn't much metal remaining between the ports and coolant passages so you can do only so much with standard porting. If you leave the auxiliary ports open at all times and combine that with a bridge port I imagine you'll end up with an engine with a very "peaky" power band.
Most people going for the most power in a naturally aspirated configuration, myself included, start with turbo motors.
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