[Piwoslaw]

I've been thinking about the freer flowing intake today. I don't know about the TDI turbo, but my HDi's turbo has variable geometry, and this lead me to a thought experiment:
The ECU is constantly checking boost pressure and tweeking the turbine's geometry to keep boost where it is supposed to be. So if the pre-turbo intake is opened up (less restrictive = lower pressure drop = higher pressure delivered to turbo) the ECU adjusts geometry to 'scoop up' less. Post-turbo pressure is the same (ECU made sure of that), but the turbo is doing less work, i.e. less work is extracted from the exhaust.

So, opening up the pre-turbo intake has the same effect as reducing exhaust back pressure. And is probably cheaper.