Regarding the high flow air filters, you should be able to get a K&N filter stateside if you really want, K&N E-4810 according to the Delica USA site. But to Steven's point, they also allow more stuff in. I would think if you are just using your van as a road-going vehicle that is absolutely fine however as a trail and exploration rig, where there is more dust flying around you want better filtration.
They've done a lot of good things with the vans in Taiwan but you also have to remember they have a very different environment than we do. It's tropical and hot af over there. It has a mountain range that is just banana's steep and most everything is paved. Offroading seems to be mostly limited to beaches, river beds, and some pits.
The conditions for lots of dust aren't as great as we have as we rip through deserts following a train of other vehicles. They also don't have the opportunity to do any true Overlanding over hundreds of miles like we do.
The use case over there is different and the design reflects that. They don't have highways that you travel 80 miles an hour on, over flat terrain, or even over mountain passes. Mountain roads are tighter, slower, and steep. Paired with the heat and humidity means that cooling is hampered.
Offroading is very different often requiring them to run big tires and lifts, even snorkels. For us, snorkels are really just a look-type thing. They don't have access to backcountry settings quite like we do nor do they require the range. Getting 250mi to a tank of fuel will get you across half the island over there.
