[GLH_TC 16V]

Headers will help out quite a bit but the design questions aren't as simple as 4-2-1, or 4-1 or cast vs. welded tubes (with or without ceramic coatings).

Headers help the flow out of the exhaust gases get out of the engine. The expanding gases turn the turbine side of the turbo then leave down the down pipe out the exhaust system.

A turbine operates on a pressure differential/change and your header design must take that into account. If the volume of your header is too big (i.e. tube diameter is too big or pipe length is too long), you will have a low pressure in the header and the turbine will not spin quickly (turbo lag). As the gas flows, it likes to go from high pressures to low pressure and the header is quite the balancing act. It goes from Cylinders->Header/Turbo->Downpipe/Exhaust. So you are balancing Cylinder pressure vs. Header Pressure and Header Pressure vs. Downpipe.

I know that this wasn't the simple answer you were looking for Airbillster, and I do tend to go overbored on technical questions but your question was quite open-ended. That being said, it is hard to get someone to cast a header for you (very expensive) and it is easier and cheaper to have one made from mandrel bent piping. I know this because I am in the process of making a turbo header (not for the PT's) and it isn't easy. The gains that I have seen thus far from other turbo header applications is approx. 30HP on a turbo car that made around 200HP with a standard manifold. Keep in mind that greater gains will be had going from a poorly design manifold to a nice turbo header.