Headers are like all other pipes on a turbo car. There is not one answer for everyone.
The basic concept of bigger pipe = slower spool and more topend pretty much applies to all pipes on a turbo car. Ie, u don't want to run 3" intercooler pipes, the biggest intercooler and an amr big header on a vf car. And u don't want to run small intercooler pipes and a small header on a big turbo car.