Headers on Turbo engines is a tricky mod to get right. You can't just bolt on a set of big headers that lets the engine breathe better like we've done in the 'Old School'. That would create turbo lag and reduce performance. They would have to allow the engine to breathe better and maintain exhaust velocity, that's a science, some even say witchcraft. And because they'll be HEADERS now rather than manifolds they will need to be made of more expensive alloys that can stand the intense heat of the turbo engine exhaust. There are grades of stainless steel that could be used but eventually the heat would cause them to crack. Inconel would be up to the task but it is expensive. Its best not to mess with them until your other mods cause the factory manifolds to be a restriction. And even then you may want to consider having the factory manifolds ported to accommodate your needs. Eventually the modding community will create a big enough need for a company to produce headers or hi-perf manifolds especially if this engine ever finds its way into being an option in Mustangs.
I know in the diesel world that manifolds and up pipes were a good upgrade when moving a lot of air with aftermarket turbos. When you got into full blown racing territory, many switch to tubular headers. On the Ecoboost I think a freer flowing cast manifold would be the best option.
Diesels are a whole different animal. They are usually single turbo and the fuel does not burn as hot. But you hit a point with 'moving A LOT of air with AFTERMARKET turbos' ... more air in means more exhaust out. There are lots of videos for those who are interested in seeing the display of heat of turbo exhaust systems.
Don't think for a minute that Ford didn't match our manifolds perfectly to our turbos. I wouldn't dream of changing them unless I was making a BIG move up in turbo size, even with a moderate increase in size the factory headers would suffice and you'd have room to move up even more with a port job.