Quote from DragonCommando :

That is correct.
What I mean is that I have never seen a completely stock vehicle run at more than 10lbs

Like I said: Smart Roadster, 1.0 bar = 14+ psi in stock!

Quote from Simpson :

In terms of reliability i was taking about ball-bearing turbos. They're not the same what you have in stock car.
As i said earlier, diesel engines have turbos with VTG, and quite short rev band. So creating flat torque curve is quite simple. How often did you see flat torque curve in petrol car? Of course you can achieve that by changing boost at higher rpms, without doing same with lower rpms, but its not the point of reprogram the ECU.

The reason why factory is putting bigger turbo is to increase reliability, bigger turbo with lower boost = less material wear, colder air intake etc. In example, VAG 1.8T
150Hp = K03, 180Hp = K03s, 210Hp = K04, 225Hp = K04, on each turbo you can make additional +~30-40Hp, but that will be 100% of the efficiency turbo could give. Don't compare diesel cars to petrol cars in terms of turbocharging, Diesel turbo won't work that good in petrol and vice-versa.

You saying the same with the vag 1.8T, petrol or diesel both can nearly always be tuned 20-30% higher then stock as long as they are turbocharged. Even with software tuning regular road-cars don't have problems with loss of boost at high rpm' s. Petrol or diesel, doesn't really matter. Well i said ths, there will always be some manufactures who did fit a turbo-charger just large-enough for the engine....

The turbo's for diesel and petrol have very different loading in terms of temperature, the basic principle how they work are identical though. VTG turbo-chargers are available now too on very limited number petrol-engines, It works great on petrol too, but the higher temperatures makes it much more difficult to design a turbo with vrt for petrol. Porsche was the first manufacturer to do this successfully on a production car. Diesel turbo' s are bit different, because the exhaust gasses have different characteristics, especially the tempature is most different.