I'm going to have to continue to disagree strongly with you. Anyone that can make 400rwhp with 5psi would clearly need some airflow mods, like a cam and/or ported heads.
Have you ever seen a vacuum gauge on an engine before? When you give it full throttle, the pressure in the intake manifold does infact go to pretty much atmospheric (0"Hg). At least it much less than 1" Hg. That doesn't mean the cylinders are filling to atmospheric pressure on each intake cycle, due to valve timing and the major restrictions at the valves and in the head, but the engine does have atmospheric pressure to draw from. On a boosted engine, you are simply give the engine a higher pressure source to draw from, but the volumetric efficiency does NOT change. You still have the same restrictions in the head and at the valves.
Yes, turbos are efficient, but they do not change the volumetric efficiency of the engine. While turbos are much more efficient than other superchargers, they still add head to the air charge when it is compressed. The best you can do is cool that air to near ambient, which is still higher than a comparable NA engine.
As great as it would be to use your logic, it is flawed. If the stock 4.6 makes ~17 hp per psi of atmoshere (260hp/14.7 psi), then you can say at 5psi, it may make 5*17 = 85hp more than stock... so 345 at the crank. If you were starting with some mild mods, making 330 hp at the crank NA (easy enough, only around 280rwhp), then your multiplier would be 22hp per psi. So the same 5psi would make 5*22=110hp more than 330, so around 440hp. That setup would be good for nearly 400 at the wheels like you were talking about... but becuase of the additional airflow mods. Same engine, but at 10psi... 10*22=220 hp more, so 550hp at the crank. This kind of calculation isn't perfect and your actual results will be LOWER due to heat and insufficient octane/too safe tune, but it gives you an idea how this works.
Turbos really do make high numbers with low boost because they are efficient. Guys on here were making about 400rwhp with 5 PSI turbo setups.
BTW You dont have 15PSI differential pressure from the air filter to motor. Atmosphere is 15 PSI above perfect vacuum, your engine doesnt pull anything like that strong a vacuum, so your differential pressure is very much lower than 15PSI.