Air flow restrictions

I have a mate with a 4.2TD patrol the same as mine (but pretty much bog standard). He is a farmer and has had a bit to do with a small device often fitted to headers/harvesters etc. This device is known by a few different names and it is fitted downstream of the vehicle's airfiltration system. It "trips" when the airflow is inhibited to a certain extent (indicating a blocked air filter in the case of heavy farm equipment). The gauge of measure is in "feet of water" and these devices come in a variety of sensitivities. The one my mate has on his Patrol is a "30 foot" type which is the most "forgiving" one he could buy. With a standard paper air filter in place, the device trips quite easily when the vehicle is under moderate acceleration or load. He has even taken the Patrol for a short test run with no filter at all (he does have a snorkel) and it still managed to trip this device....but only under full acceleration. Note: he only did this on an open country road after a shower of rain so there was virtually no chance of any air born dust. It is not something I would try myself!
Just wondering if anybody has any thoughts? The device he is running is made by Donaldson, but I can't find any reference to them on their web site. He has spoken to the Donaldson blokes and they can't believe a 4.2 TD patrol could trip a 30 foot model.....it means there is an intake restriction of high proportions. It's just made me wonder as I'm typing this, whether he might have a piece of foam rubber just before the air filter box....I think Nissan have this fitted in the inner guard and it should be removed when/if a snorkel is fitted.
Any info or other people's experience would be appreciated. We all tend to talk about getting our vehicles breathing better (by whacking on 3" exhaust systems etc), but all that is gunna be useless if the bloody thing can't suck in enough air to start the whole "breathing" process.
Cheers
Roachie