[HDi fun]

I'll take that at face value and thanks for raising this point. In my experience modern diesels have very pretty freely flowing exhausts. The problem with the 307 we're discussing is that in all likelyhood it has a DPF.

This will be the most restrictive exhaust component, apart from the turbocharger itself.

Turbo diesel engines do not make significant use of exhaust scavenging to create partial vacuum in the intake part of the cycle. There's no real need anyway because there's no throttle on a diesel engine so it's alway breathing unimpeded atmospheric air at worst. Once the supercharger gets into its stride the air is rammed in by that very effectively.

There will be room for improvement but when you consider that a modern diesel engine does its best work between about 1500rpm and 4500rpm at best the need for massive valve overlap is not there. Nothing is opening and closing fast enough for the opening/closing periods to really impede gas flow.

As for panel filter replacements i totally agree with Jarrus. I can't see the need. OEM filters are dead cheap. Cheap enough to replace every couple of months if necessary and a lot less fagging around with the cleaning/oiling process required of a supposedly high performance alternative.