Its quite simple really in a naturally aspirated engine with stock exhaust, the valve overlap on the engine allows the exhaust volume moving out of the head to scavenge the cylinder better allowing a greater intake volume to be pulled into the cylinder, it is timed up so that the valve closes before the intake charge begins to leave the exhaust valve. When you put headers on an NA car you improve the scavenging of the exhaust improving the intake charge to the point that just a little intake volume goes into the exhaust before the valve closes but the cylinder is filled with more intake charge regardless so it makes more power. On a supercharged car the intake charge is pressurized so that once the exhaust has vented enough that the cylinder pressure is equal to the intake pressure intake charge begins to flood out the exhaust valve long before it is closed, with a restrictor like a factory cat, or running a smaller exhaust diameter(2-2.5") with test pipes you allow manifold pressure to rise right at the critical point where the intake charge would over pressure the cylinder and flood out the open exhaust valve so the manifold back pressure holds the charge in.
This has been true for long before the 370z even came out, every supercharger kit manufacturer will warn about headers causing a loss in boost pressure, the fact is with a supercharger and the locked in cam timing the Z will make more power per cfm pumped into the engine with a restrictive exhaust. headers plus cam timing the way it is the engine wastes a large percentage of intake charge, so the header manufacturer tells you to get a bigger blower so that even though 10% of the intake charge is flying out the exhaust you will get your power back by shoving more in. The supercharger manufacturer tells you to put a more restrictive exhaust on the car so you waste less and get more pressure and power for the amount you spin the charger. In the end the both do the same thing you just have to pick whether you want headers and a more wasteful setup or a smaller setup that's more efficient.
Now if you can change the valve timing, grab your self some headers and turn the overlap down and enjoy the best of both worlds. Like I said none of this is new it's neither companies fault it's just the way it is, thats why back in the 60's a thing called a blower cam was invented.
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