I will add from my experience in engine building for almost a while. Build a turbo engine like an allmotor engine, exception is slightly larger exhaust valve and steel valve. I usually run titanium intake valve since it sees less heat and it's bigger and my motors like to spin, so Ti helps with that.
The closer you can get boost : backpressure to equal 1:1 or 1:0.8 ( which is a dream for some, but not all) the more valve overlap you can run without congestion. Imagine 1:1 creates no push or pull during overlap. If it's 1:0.8 the exhaust becomes a vacuum.
Turbine A/R plays a major role in backpressure. What people don't get is, if you have an effecient system, it can carry a larger A/R.
Your motor will produce more horsepower with the same boost level, because it's not ingesting exhaust gase's that are unburnable on the intake stroke. And displaces the cylinders volume, depriving that bit of volume of having that much more clean air and fuel from intake port. Thus reducing your volumetric efficency.
Biggest hinderence is long exhaust systems on turbine outlet, increases back pressure, and restrictive or long path on the compressor outlet. I prefer an airbox for compressor inlet, air is much more laminar and if well placed cooler.
Exhaust headers I like to match the port exit for CSA and 1St step out a few inches depending on exhaust total duration, and step out at every 11" @ a 10% increase in CSA at each step. I do that until I reach the turbine inlet.
Twin Scroll is a must on 6 cylinder single turbo. Merge collectors I like 5° included angle leading into the turbine. This makes for a early, fast power and maintains drive all the way up.
Also tighten up piston to head clearance a bit, lay back the valve pocket reliefs on the intake side of the piston. Your piston acts as the floor of the port at TDC on the intake stroke. Those ridges create a lot of turbulence and back flow. Raise the intake ports from the valve guide leading up to the intake manifold , fill the floors for what your removed on the roof and smudge more. Fill the roof of the port between valve seat and guide. All with quality epoxy not JB weld.
Make sure your intake match the ports.
Do half of this stuff and you'll be amazed how well it responds.
I've built a lot of engines, but production BMW motors are jewels.
As a note I moch up the motor with cam timing at 0° for intake and exhaust Vanos. Works out around let's say 105 In LC and 105 Ex LC. I then move the cams around and measure how far I can go each way until things become tight. That way when I program Vanos I don't get in trouble guessing instead of knowing.
You have to check Piston to valve clearance and valve to valve clearance when your are trying things other than OEM.
I hope I offered value, I know most are hush hush. I'm a privateer grassroots.
