Your your snippet actually concludes what I am trying to say....read on if you so wish

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yer want to get as much air in as possible so you can burn as much fuel as possible so therefore you don't want anything that will restrict that
Of course because at the end of the day it's a combustion engine burning air and fuel. But it is only up to a point of what the engine, management, etc etc can take.
Can't quite describe what I am trying to put across. Back pressure itself is not needed...infact you wan't low back pressure. However... what many people mean by back pressure is how free-flowing the exhaust is...and this is the tract that I am going down. A bigger bore/shorter/straighter/un-muffled exhaust doesn't necessarily mean that the gases will get out of it quicker. Of course it could improve performance, up to a certain point, but if you go too big bore etc for your state of tune it will impede performance....
Why? Well....
Ignore backpressure for a minute...as we know that isn't what we are really talking about....The ideal exhaust system will have a high gas flow but maintaining the ability for a vacuum to develop behind the closed exhaust valve (or the "scavenging" effect, as you pasted above). If your system is too big bore/free flowing/short/etc for the timing/tune/etc for your engine, the flow will be be slower and the scavenging not as good. Does that make sense?
I understand where you are going about the drag cars running no exhaust, just headers, and that link for the cut off device etc...but these are designed for cars with a huge amount of gas flow....so they don't need suffer with the normally associated problems. And infact, people running these drag cars dont just get the biggest headers they can get (which would back up the bigger bore the better arguement)...it all gets matched to the engine.
As for the application on the TD engine above...being a turbo the effect of having a bigger bore/free flowing/etc exhaust wont be as great when compared to a NA engine because turbo engines have a lot more gas flow than a NA engine and as suggested adjusting the fuel mixture could probably sort it, if that is needed at all (and maybe not at all if he is not going to a big bore). If it is only a tiny turbo on the engine then you may need to do more to make it run right, because you will have less gas flow.