yeah I know it was a lot lol I guess I should have asked whats up with the parts nobody talks about the abnormal stuff not turbos or intakes or intercoolers but spark plugs differential stuff, manifold spacers , fuel rails
I thought i'd give another crack at answering your broad question without delving too much into one specific question.
Most of the things you mentioned are not big bang-for-the-buck mods. They are the kinds of things you do when you want to build the car right, and make it the best it can be given other limitations (turbo size). You'll get much more HP out of dropping in a bigger turbo than you will by adding a turbo blanket, heat wrapping your headers, upgrading your fuel rails, doing a tgv delete, AND adding phenolic spacers--but they'll cost about the same. Consequently, they get ignored by most people because they can spend their limited budget elsewhere and get "more power" on a dyno plot.
Does that mean they're not important? No! They're the subtle things that separate a typical build from a good build. They're the kind of things that may result in more real-world power (with heat soak), or power delivery and spool, or even margin of safety.
I again, encourage you to ask more questions, just break them up a bit.
For instance, you piqued my interest in fuel rails--i dont know much about the limitations of the stock ones, and what you gain from going to an aftermarket solution. More flow capacity? more even fuel to each cylinder?