I have measured the pressure drop at full throttle on a clean 3.9 OEM filter at about 11 inches of water including the snorkel piping, and interestingly to me anyway about 0.5 inches positive at 80Kmh.
In other words a poofteenth.
So either the Tdi filter is an overkill, or they expect tdis to be used on dirt roads more.
I can tell you " from the heart of my bottom" that there is no room to put a Tdi filter under the bonnet of my 3.9 anyway.
I tried modifying a Commode door filter and that wouldn't even fit.
Regards Philip A