There are two things happening in the exhaust system and they are often confused.
1. Flow vs pressure drop
2. Pulse tuning

It is unfortunate but people get them confused time after time. The major reason full length headers are better than most exhaust manifolds is pulse tuning. A well-designed full length tube header adds torque below the torque peak as well as above.
Exhaust manifolds do not have long enough runners for the pulse tuning to work and therefore cost power.
Exhaust manifolds tend to have short, cramped runners. The high performance manifolds may have smoother bends and make an effort to connect runners together in such a way that interference is minimized. So, they can flow more exhaust. But even if they flow same as a long tube header, the long tube header will make more power.

There isn't any magic number of horsepower that is maximum for a particular exhaust setup. Look at AndyF's work with his 470 stroker. On this particular engine, running stock HP exhaust manifolds, he was able to get quite respectable hp numbers. In his case the MP 0.528 cam seemed to be optimum. You might look at the parameters of that cam as a starting point for your engine.

In the case of exhaust pipes being limited in size the H or X pipe is clearly beneficial. It lets the exhaust flow "see" both exhaust pipes.

For racing if the class allowed headers I'd sure try to use them, even if the rest of the pipe was 2". The benefits of the pulse tuning are too good to pass up.

R.