hcl, fuel flow restriction determines your max power (and torque @ rpm) that was my main point. Take a look at some BSFC maps:
https://www.google.si/search?q=bsfc+...w=1309&bih=895
They look all similar, no mater if diesel or petrol and probably every competitor would like to optimize their engine so that it achieves their max power at max fuel flow in that sweet spot of efficiency. The end product is obviously a compromise of size/weight and efficiency.
I try to get all the boost preasure, wastegate, exhaust preassure, and the only logic that I can get is if you design engine in a way that there is practically no exhaust pressure left (eg. atkinson cycle) then you are getting good efficiency. The fact that downsized turbo petrol must use waste gate that actually means that there is energy thrown out the exhaust (captured by MGU-H). I think there is a path in the middle, meaning that you can design a bigger engine with smaller turbo that wouldn't need much (or any) wastegate and I think Audi is on that path. If not they would definitely be running the MGU-H unit.
Direct injection can give you lean burn under lighter engine load, but this is still to light load to get peak efficiency, DI is also good to maximize the engine power but this means running on rich fuel ratio and your efficiency goes down the drain. Toyota has D4-S system in some of their road cars (GT86, IS 300h), the system combines direct and port injection, why do you think they kept the port injection?