All but the most highly engineered overbuilt turbo systems see more backpressure in the exhaust manifold than boost pressure in the intake tract. Even those systems are tuned for a particular boost/rpm range.

So many people think that turbos are free horsepower. They are NOT free. The engine still has to work the turbos. That results in slower velocity in the exhaust manifold while the air is trying to cram it's way past the choke point in the turbine housing.

Fancy headers can make more power than a decent common manifold but the difference on a moderate street engine is not that big of a deal.

Unless you're planning on running more than 700 hp out of a common V8 then you're not going to see a 20 hp difference with pretty headers.